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.
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.
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.
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!
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.
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.
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.)
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.
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.
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.
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.
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.
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.
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.
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?
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.