'94 Corvette Dangerous throttle surge.

[brewha]

My coworker's car has a dangerous tendency to occasionally surge while idling. It's a 1994 Corvette automatic with the 300HP V8. It doesn't do it very often and only does it after it gets warm.

It's bad enough that it can overpower the brakes and he has had to throw it into neutral to stop the car from taking off through a red light. He has checked the computer and it isn't throwing any error codes.

I told him that something like that needs to be inspected by a dealer to see if there's any recalls or service bulletins. He's planning on taking it in next week, but I'm curious to see if any of the knowledgeable car guys (or gals) would have some insight as to what might be going on.

So, general question:
What's wrong with this car?

[jz78817]

Vacuum leak? I can't remember if that generation of vette was speed-density or mass-airflow, it if it's speed-density then a vacuum leak can cause that type of surge.

[Nunzio Tavulari]

Assuming that the brakes are in proper working order, there is no car that can overcome a brake pedal pushed to the floor. It's been proven time and time again, most recently by Toyota a year or two ago. You could be doing 100mph and mash both pedals at the same time and you will decelerate to a stop.

[md2000]

I recall driving a friend's Ford (1970 vintage, IIRC) many years ago, and the pedal had an annoying habit of sticking. Very dangerous. After one episode where it started racing and I had to turn off the ignition, I looked under the hood.

The cable sheath for the acelerator had worn through. (old car). Essentially, the cable sheath was split for about a foot or more where it arced from the carburetor to the lower firewall. Sometimes, if you pushed the pedal too suddenly, the cable would pop out of the sheath at that arc; the sheath would then grip it tight enough for a while that the carb stuck fairly open. I suppose the vibration of the engine eventually let the cable pop back in the sheath if there was no tension (no foot on pedal).

New cable fixed his problem. The problem was not obvious until the car was examined in the "stucK" position, because the wear-through was on the underside of the cable. It looked just fine from on top.

Of course, if your problem is a computer, good luck!

[brewha]

I would think that a vacuum leak would show up as an error on the computer diagnostics. From the way he described it to me, the engine revved right up to 5,000 RPMs. For that, you need fuel air and ignition in the right amounts.

I suspect that there's a potentiometer that reads the accelerator pedal position and that it's occasionally sending the signal to the computer that the pedal is being depressed. Pure, speculation, but it's all I got.

[brewha]

Assuming that the brakes are in proper working order, there is no car that can overcome a brake pedal pushed to the floor. It's been proven time and time again, most recently by Toyota a year or two ago. You could be doing 100mph and mash both pedals at the same time and you will decelerate to a stop.


You realize that a 300 HP V8 reacts to throttle a little different then a Prius, right?

[jz78817]

You realize that a 300 HP V8 reacts to throttle a little different then a Prius, right?

No, he's right. Otherwise, brake torquing wouldn't work.

If there is a vacuum leak, it could be causing the brake booster to work less effectively (or not at all) which means he simply can't push down on the pedal hard enough. Assuming the brake hardware at all three corners is working properly, that is.

I would think that a vacuum leak would show up as an error on the computer diagnostics. From the way he described it to me, the engine revved right up to 5,000 RPMs. For that, you need fuel air and ignition in the right amounts.

As I recall, the C4 never had throttle by wire, it has a cable. So if the thing is revving like that without him pushing the pedal, then either something in the throttle body is broken in a strange way, or there's a vacuum leak.

see, the thing about speed-density engine management is that it calculates the volume of air the engine is taking in by looking at the pressure/vacuum in the intake manifold, the temperature of the air, and other inputs. If extra air gets into the system via a leak downstream of the throttle, all the PCM sees is that the intake manifold pressure went up and it happily adds more fuel to match. It's technically "metered air." Now, I'd expect that if the manifold pressure is out of whack with the throttle position it should be throwing codes, but maybe it's not out of whack "enough."

(on a car with mass-airflow engine management, or one with a carburetor, air taken in via a vacuum leak is "unmetered air" and will usually cause poor/no running, stumbles, and so on since the extra air leans out the mixture and the PCM/carburetor can't account for it.)

[Rick]

Two things strange about this story.
First off:
One of the standard tests when checking automatic transmissions is called a stall test.
To do this test the technician applies the brakes and while in drive floors the gas to observe the RPMs Typically the engine will rev to between about 2200 RPM and 3,000 RPM depending on the make and model of the car.
Now since I have never seen a transmission shop with a hole blown in the back wall from doing stall tests, Nunzio Tavulari is right. The brakes are stronger than the throttle.
Secondly, See what I said about the stall test and the RPMS? I highly doubt that any stock car has a 5,000 RPM converter installed. Maybe some drag race cars, but no stock car.
I doubt the OP co-workers story.

[brewha]

I talked to the guy again. He was sitting at a light. When the car started to rev, he popped it into neutral and that's when the tach jumped up to 5K.

What would cause the engine to spontaneously rev? Can we ignore the brakes?

[Rick]

Ok now this is starting to make a bit more sense. But not lots more.
Ok does the car still idle at 5k?
If yes I would look toward a vacuum leak. Or possibly a stuck throttle.
If everything is back to normal now, either an intermittent air leak or demonic possession.

[brewha]

It runs and drives normal most of the time. Occasionally, the engine will surge momentarily then settle back down to a normal idle. But, as it is an intermittent problem, it's difficult to diagnose.

I was hoping that someone "in the know" may have heard about a service bulletin or have had experience with this kind of thing happening.

For now, I'm going with demonic possession.

[Morgenstern]

. . .

It's bad enough that it can overpower the brakes and he has had to throw it into neutral to stop the car from taking off through a red light. He has checked the computer and it isn't throwing any error codes.

. . .

Assuming his brakes are in good working order, I can assure you that they are more than capable of keeping the car from surging through a red light when the engine is at even a very fast idle. Throwing it into neutral won't hurt it, but his fear is unfounded.

In fact, you can stomp hard on the brakes, and the throttle, and the car will remain stationary as the back tire(s) spin.

[jz78817]

It runs and drives normal most of the time. Occasionally, the engine will surge momentarily then settle back down to a normal idle. But, as it is an intermittent problem, it's difficult to diagnose.

I was hoping that someone "in the know" may have heard about a service bulletin or have had experience with this kind of thing happening.

For now, I'm going with demonic possession.

dude, the car's almost 20 years old. it could be anything. Like Rick and I have been saying, I've seen this kind of thing caused by a vacuum leak. Hell, I'd look at the check valve on the brake booster, or the vacuum hose leading to it. Those things are usually plastic and degrade and crack over time. It would explain the inability to hold the car with the brakes as well.

[brewha]

For fucks sake. This isn't about brakes.

[jz78817]

For fucks sake. This isn't about brakes.

they're related. the brake booster uses manifold vacuum to increase the effective pedal force. If there's a big enough vacuum leak, then the brake booster can't apply as much assist and the driver may actually have trouble holding the car at the stop.

you'd be surprised at how hard you have to stomp on the pedal in a car with power brakes after loss of power assist.

[Rick]

One of my first thoughts was the one way valve in the brake booster line is sticking open.
This would give shitty brakes and a very high idle.

[Gary T]

For fucks sake. This isn't about brakes.
It's bad enough that it can overpower the brakes...With this statement from the OP and several knowledgeable posters stating that the engine cannot overpower properly working brakes, it looks to me like it is about brakes, in part.

[Rick]

So if you were not planning to listen to the advice given in this thread why did you take the time to write the OP?
If you are expecting someone to come and identify where the magic screw is on a Vette that when turned 1/4 turn fixes everything, don't hold your breath.
Go back and read post #7. I am almost certain that some type of vacuum leak is the root of the issue that post explains why. And you are not looking for a little tiny vacuum leak, it would have to be fairly large to do what is being described. Like the hose to the brake booster, or maybe the brake booster itself is leaking. That could explain the brakes not feeling very strong and booster can come and go.
Does you co-worker ever hear any hissing noises inside the car while driving?

[Nunzio Tavulari]

You realize that a 300 HP V8 reacts to throttle a little different then a Prius, right?You realize a Camry V6 is 268 hp right? It was a big story, more deaths than the Pinto.

[brewha]

I do appreciate the advice and actually did miss the very informative post #7. I don't understand how a vacuum leak could be intermittent, but aside from that, it seems to be a plausible problem.

Clearly, I am only looking for a magic screw.

Obviously, it is absurd that I was expect anyone to chime in and say "I had that happen to my (corvette or chevy 350 powered car) and this is what caused it." Or, "I happen to work a a dealership and there is (or even isn't would be helpful) this service bulletin currently out for a vacuum leak."

Of course arguments about brakes is what I got. I should have expected as much.

[jz78817]

I do appreciate the advice and actually did miss the very informative post #7. I don't understand how a vacuum leak could be intermittent, but aside from that, it seems to be a plausible problem.

Clearly, I am only looking for a magic screw.

Well, if you're not going to read informative posts, why are you complaining?

Obviously, it is absurd that I was expect anyone to chime in and say "I had that happen to my (corvette or chevy 350 powered car) and this is what caused it." Or, "I happen to work a a dealership and there is (or even isn't would be helpful) this service bulletin currently out for a vacuum leak."

It's not necessarily absurd, but it's unrealistic. That car is nearly 20 years old. I doubt any of them have shown up in dealer service bays for quite some time.

Of course arguments about brakes is what I got. I should have expected as much.

If you would bitch less and read more, maybe you'd understand why the brake discussion is germane. There's not a car on the market that can overpower its (properly functioning) brakes from a stop. Not one. NOT ONE. If the engine is racing and he can't keep the car stopped, then something is reducing the effectiveness of the brakes. E.g. The brake booster isn't getting sufficient vacuum because of a vacuum leak.

[brewha]

And, if you would fuck off more and suck less the world would be a better place.

Seriously, why is this such a hot topic debate? I asked if anyone had any experience with this type of issue. What I got was people who obviously had no first hand experience but wanna wave their dicks around and shown how amazing their mechanical ability is. Why is everyone so fucking hung up on the brakes? I've already explained that I clarified the issue with the car's owner and the car can not in fact overpower the brakes. If people can't read the relevant posts I have no use for this thread. NOT ONE.

To the 735 people who read the OP and decided that they had no idea what could be the issue - thanks. Your non response was the correct one.

[JBDivmstr]

...
To the 735 people who read the OP and decided that they had no idea what could be the issue - thanks. Your non response was the correct one.

You're quite welcome. :D



Sorry, I just couldn't resist.

[brewha]

No snark apologies necessary, JBD. I genuinely thank you.

[JBDivmstr]

::slight hijack::
On a mostly unrelated note... Many, many moons ago I had a '70 Chevy p/u that developed a broken motor mount on one side, (I forget which) and when you romped down on the gas, the motor would jump up and the throttle would stick until the forces involved allowed the motor to settle back down, at which point the throttle would 'unstick' itself.

When you're only 17 years old and just learned how to drive, that kind of shit is scary, to say the least! :eek:

[Gary T]

I've already explained that I clarified the issue with the car's owner and the car can not in fact overpower the brakes.I don't see any explanation that you clarified the issue with the owner.

I don't follow that you "explained...the car can not...overpower the brakes." Explained to whom? Did you mean to say you explained that the car did not overpower the brakes? When? Where?

I did see this: I talked to the guy again. He was sitting at a light. When the car started to rev, he popped it into neutral and that's when the tach jumped up to 5K. Is this what you're referring to? It doesn't contradict the statement in the OP that he "had to throw it into neutral to stop the car from taking off through a red light."

If you're trying to say you told us that the statement in the OP was wrong, and that there never was any kind of issue with the brakes, and that's why you're upset with the replies, then I have to reply that you did NOT in fact tell us that said statement was wrong.

If people can't read the relevant posts I have no use for this thread.We can read what's written. We can't read what's left out.

[brewha]

So if you were not planning to listen to the advice given in this thread why did you take the time to write the OP?

How else would I be able to argue the merits of the modern brake? Or to discuss the horsepower of a toyota camry?

Sorry, that was a bit snarky. Here's the real answer:

why did you take the time to write the OP?

Honestly, I haven't a fucking clue.

If a passing mod wishes to close this disaster, feel free. I won't be back. I guess you guys can start your own thread about brakes and toyotas if you want to discuss it further.

::walks off muttering incoherently::

[henshao]

I have a pair of F-bodies from that era. LT1 powered cars like the 94 corvette. the throttle blade is linked to the pedal by a cable. based on the strength of this particular surge, it sounds like the throttle is being opened. a vacuum leak would have to be immense to take a car up near redline like this.

could be the cruise control servo twitching, as it was mentioned earlier the car is nearly 20 years old.

[AZBlackC4]

I have a 94 corvette with an auto and have spent alot of time lately tracking down the source of the very same symptom. After alot of research and posting on my corvette forum, I am getting close to the solution and a better understanding of what causes this. I'll try to make it short. And you're right, it doesn't have anything to do with the brakes.

My car was surging badly and stumbling at idle after it got warm. I replaced the plugs, wires and fuel filter and it ran pretty well until it got warm and then it would surge. Not quite as bad as before but it was still there. I checked the plugs and wires in the dark while it was runing, and found a loose plug wire and a cracked plug. Replaced the plug and reinstalled the wire, and it ran much better, but still surged a bit when warm. I then, cleaned the MAF sensor, the Idle air control valve and the throttle position sensor. Again, it ran well and seemed to have better driveability, but still surged when hot. I checked all the vacuum lines, sprayed carb cleaner around the vaccum lines, etc when running and all vaccum lines appear to be good. I then cleaned the EGR valve. Ran well, but still surges. But not as bad. Something is still causing the computer to increase the idle speed. I then replaced both front O2 sensors. The driveability got much better. The engine accelerates smoother and the idle is better, but there is still a surge and stumble at idle and a hesitation when accelerating when the engine or engine bay gets hot. I have also replaced the MAP sensor, which can also cause this problem because it is directly related to manifold vacuum pressure. Problem is still there. When the engine gets hot and the outside temperature is hot, like over 90 degrees, the engine also hesitates and looses power when accelerating. This is definitely a temperature sensitive problem, and I think it is more related to the engine bay and ambient temperature being hot, than just the engine warming up.
I think I have it narrowed down to the fuel pressure regulator. I am replacing that next to see if it clears up. I read on my corvette forum that someone had the same symptom and it wasn't because of the engine getting hot, but the engine bay and the fuel pressure regulator getting hot. If the diaphram inside the fuel pressure regulator has a leak, it will show up when it gets hot and expands to open up the leak. He replaced the fuel pressure regulator, and the problem went away.
As you can see, I have replaced or cleaned quite a few sensors and parts to find what the root of the problem is. The reason I have replaced or cleaned all the items I mentioned is because any one of them can cause something like this to happen, or a combination of the above. My next move will be to replace the ignition coil and module. These can also cause a variation of this to happen when hot due to the expansion of a small crack.
This car is 18 years old and all of these sensors need to be replaced anyway. Most of what I have mentioned is not very expensive to replace and can be done by yourself. The spark plugs and wires are not easy though, and costly at a shop.
I have heard that a varitaiton of this problem could even be due to the Opti spark failing, which is not cheap to replace.
After I replace the fuel pressure regulator, I'll post if it clears up the problem. If it does, then I'll know that it was the main source of the problem to begin with.
Let me know if you have any questions Brewha, I'll try to answer what I can. This is a frustrating problem, that is hard to track down and if I find out what caused it in my car, I'll sure let every body know what it is so you don't have to pull all your hair out trying to figure it out! :smack:

[AZBlackC4]

I thought about your specific surging problem a little more, and I think yours may be related to the Idle Air Control valve. The IAC. This little device controls the idle on the LT1 engine. I'd start with that. Pull it off, clean it and see if the problem goes away. If it doesn't clear up after cleaning it, the IAC may need to be replaced or one of the other sensors, vacuum lines, etc. I'd check the Throttle position sensor too, clean it and see if that helps. As I said before, the car is 18 years old and all of the sensors either need to be cleaned or replaced by now. :(

[henshao]

A stumble could easily be the OptiSpark, i've had to replace quite a few of those. But a surge, not as likely. A faulty IAC could also do it. I don't agree that all the sensors need to be cleaned or replaced by now, however... There are a truckload of sensors on these cars and the vast majority of them would have to be working for the car to be driveable. 99% of them are still working on my two LT1 powered cars, although many of those have failed over the years and been replaced.

[AZBlackC4]

I agree Henshao, you don't necessarily HAVE to replace all the sensors. I made that statement more from my "point of view" from working on my car and the driveability. Clearly, most of the sensors on my car are functioning enough to allow the car to run fairly well most of the time. What I noticed when I started to try to track down the problem, is that after I replaced or cleaned any one of the sensors i mentioned, the car ran a little better. After I replaced the fuel filter, it ran better, the plugs and wires, it ran better, cleaned the EGR valve, it ran better, replaced the O2 sensors, it ran much better but there's still a small surge and stumble at idle. In other words, the parts I replaced were functioning enough for the car to run, but rather poorly. When I replaced the O2 sensors, there was no "check engine" light on, and the car ran okay. After I replaced them, there was a noticeable difference in the throttle response and acceleration.

Brewha can do whatever he wants with his car, but from my perspective, I'm going to try to go through my car and replace most or all of the sensors and ignition parts including the optispark, but on my time and terms, not when something completely fails and I have no choice. (like the optispark!) Ugh.
Given the fact that the car is 18 years old, I just think it's prudent to replace these parts. I'm looking at all of the connectors too. My MAP sensor connector was crumbling when I replaced the MAP sensor, and i replaced it too.

[mack]

My coworker's car has a dangerous tendency to occasionally surge while idling. It's a 1994 Corvette automatic with the 300HP V8. It doesn't do it very often and only does it after it gets warm.

It's bad enough that it can overpower the brakes and he has had to throw it into neutral to stop the car from taking off through a red light. He has checked the computer and it isn't throwing any error codes.

I told him that something like that needs to be inspected by a dealer to see if there's any recalls or service bulletins. He's planning on taking it in next week, but I'm curious to see if any of the knowledgeable car guys (or gals) would have some insight as to what might be going on.

So, general question:
What's wrong with this car?
I had a Ford Escort that used to do that. Scary as hell. After a bunch of mechanics I scoured the internet and found the problem - it was the idle air bypass valve. It was a $45 dollar part that bolted to the top of the throttle body on the Escort. Or it could be cleaned with carburetor cleaner.

ETA: What AZBlackC4 said.