2001 Dodge Dakota. 4.7 V8 with Automatic and 4WD. 103,000 miles.
CEL = Check Engine Light
Runs great. Starts great. Consistent. Always was. Never hesitates or stumbles. No signs of problems.
Driving along highway, around 60-70 MPH, under light load/throttle (load being the load on engine) and the vehicle will jerk/buck momentarily, with the CEL coming on to a steady glow almost instantly.
Engine management system (I have noticed) goes into protection mode at this point, and truck runs fine, but engine is not permitted above 2600 RPMs. This protection mode is cleared by turning engine off and restarting. Quite frankly, this is enough RPM to get off the road, but not enough to accelerate or be safe in continued driving.
Generally, about two cold/warm/cold engine cycles later (usually the next day) the CEL is off after ignition.
This problem goes away, then comes back about a tank later. It seems to happen most often when I am under light throttle… cruising… engine under hardly any load. I can almost sense when it’s coming.
If I run a whole tank with ‘spirited driving’, it doesn’t happen. If I tend to cruise at light throttle and load I can sense it coming… then it bucks for .5 secs w/ the CEL coming on.
Any of the SDMB auto gurus wanna take a stab at this one?
Getting the computer scanned would be the best bet, but if we’re going to make guesses, I guess the Throttle Position Sensor. The TPS seem to develope a bad spot right above idle.
When you are driving in ‘spririted mode’, you are likely not in the ‘bad spot’ long enough to cause a problem.
I am not aware of a “protected mode” that the computer can put to limit the engine rpm’s (and why would there be?). What happens likely is that a sensor is intermittently failing and when it does the engine disregards its input and starts using some default values, which depending on which sensor, still will allow the car to run to a certain degree.
Since not getting past 2.6k rpms is a pretty big problem, I’ll bet it’s a very important sensor-- probably something like an airflow or manifold pressure sensor (I’m not intimately familiar with Chrysler FI systems). Probably not a TPS sensor because these will only affect the engine in transitional states (i.e. accelerating and decelerating)-- without it’s input the car will still slowly-but-surely stumble up to speed. It’s also probably not something like a crank position sensor, since the computer can’t guess it’s value so if it goes out, you’re stuck.
Getting the codes read would be nice, but may not yield definitive answers. There is also always the possibility it’s something weird like an intermittently failing fuel pump or a bad distributor or something. But getting the codes read would probably be a good start.
It never ceases to amaze me how much people overestimate the usefulness of OBD-II. The cynical part of me wants to say that the reason why places like Autozone read codes for free is because they know it leads to the sale of a lot of unnecessary (and expensive) sensors.
I wasn’t really implying any malfeasance on the part of Autozone (it’s quite nice of them to provide that service), just that often the trouble code isn’t really that helpful and can lead to unnecessary part replacement if one doesn’t do some actual under the hood diagnosis as well.
I have a Dodge Ram, and a code reader.
I have been in what i refer to as a “Limp Mode” and what caused that was a fouled plug:dubious:
The limp mode cleared itself after I limped about 10miles home:smack:
The latest malfunction was what i diagnosed as a throttle position sensor. There was no CEL but every time i let off the throttle the engine would die immediately.
I ordered the part and drove 2 days (i had to keep a size 13 on the throttle at all times) and it cleared up the day i was planning to install the new sensor. That sensor is sitting in the cup holder now for 1 month and I just know that as soon as I return it (I work part time at CarQuest) it will fail again. And my code reader is at the parts counter for anyone to use. Its a simple to use basic reader that turns on reads the code and even turns itself off. The most common error code i see is related to a miss fire CEL.
I should have sprayed in some disclaimers, as I know all too well the intrinsic value of OBD, but I also know they don’t completely eliminate the guess work (potentially expensive, too).
I believe I go into ‘limp’ mode when the engine is not allowed over 2600 RPM.
I did some pecking around, and this is the reason I posted here at SDMB: I went with the TPS lead offered here, and it appears I am (from a number of boards) a classic case of a bad TPS (based on some other minor quirks leading to a preponderance of the evidence). I am also a classic case of “Hey, if I get the codes, I would probably chase my tail first, spending hundreds when I need a 25 plug-in, DIY part.”
I am auto savy enough to be cautious and informed before getting sucked into the code vortex.
I don’t believe such a mode exists. What possible engine problem could there be where the computer would need to limit the engine speed like this? I think the inability to accellerate past 2600 is a direct symptom of the problem.
Like I said above, I don’t think this is a classic case of TPS. The engine primarilly uses either a manifold pressure sensor or an airflow sensor to determine the engine load and thus how much fuel to use at that engine speed. All the TPS is there for is so when you suddenly accellerate or deccellerate, the computer can react without having to wait for the air flow and/or manifold pressure state to change. So thus the “classic case” of a bad TPS is you stomp on the gas and the engine stumbles or bogs down but eventually chugs up to speed, or alternatively, the idle will be erratic since the computer always thinks you’re starting to accellerate. Neither of these sound like the situation here, and I don’t see how a bad TPS would cause you to hit a brick-wall engine speed.
A number of vehicles include limp modes (Saturn, Subaru, and Harley to name a few off the top of my head). These limp modes on the cars tend to engage when a integral sensor dies but there is still enough data to run the car (as opposed to a CEL light coming on for an O2 sensor, your emisions go to hell, but the run-ability is usually fine). For HD the limp mode is for the 07 up touring bikes that came with the ETC throttle, when the TGS (twist grip sensor), ECM (for some errors), or the TP Solenoid have issues the ECM holds the engine at 1500RPM so you can try to limp it to service (this means you better be really good with the gears and the clutch!).
Not true. The classic case of a bad TPS is a bad spot. The TPS is like a volume knob, varying the signals resistance depending in the throttle position. If you push the throttle quickly past the bad spot, you are not likely to notice, or notice just a slight hesitation. But if you are driving at a speed which leaves the throttle in the bad spot for more than an instant(exactly what the OP describes), it will stall, or hesitate. The TPS may work perfectly at all other throttle setting but that one.
It’s called “limp home” mode, and it most certainly does exist. Its purpose is to prevent possible damage that could be caused by normal vigorous driving when certain malfunctions are present.
From here: “As such, many ECU units are fitted with a ‘limp home mode’. When it detects that something semi-serious is wrong, it enters the limp mode, which means that the engine won’t rev beyond a pre-programmed speed, you can’t accelerate very fast, or you may indeed be speed-restricted to 40 mph or similar.”
From here: "The strategy behind the “Limp Home” mode of operation is to ensure that the engine is still operational in case of a worst case scenario…But even more so to PROTECT the engine itself.
"If a critical sensor, like an air flow meter, is faulty, then the engine is at risk of running too lean…[which] could potentially burn a hole through your pistons or weld the pistons to the sleeve of your engine-block.
“For this reason the computer compensates by making sure that the engine is ALWAYS running far too rich. Hence it wont damage your car whilst driving in this mode because it is intended to protect your engine.”
By limp-home mode, I mean a performance limitation intentionally created by the computer, as opposed to the computer running on default values for a certain sensor in which case the performance is simply limited by the lack of data. I’m quite familiar with limp-home modes in the context of electronic-controlled transmissions restricting maximum speed or locking out certain gears but I’ve never seen one for an engine. I guess it does sound like they do exist on newer cars, but I’m still not sure that’s what’s happening here.
The important sentence in the allpar link is: “While at partial throttle positions, the logic module uses the TPS sensor signals to improve reaction times of air/fuel mixture adjustments.”
Perhaps a rough spot is a more “classic” TPS problem than what I’ve described, but even with a rough spot, it will eventually accelerate up to full speed as the MAF/MAP sensor catches up. A bad TPS won’t stop the engine from getting past a certain RPM.
The engine is clearly ‘rev limited’, a feature I am very familiar with, when the CEL comes on after the vehicle bucks momentarily. Best I can tell, this is ‘limp mode’. It’s a “hard” stop of the engine, at about 2550 to 2600 RPM. Power is cut momentarily, and returns, only to be cut again (wash, rinse, repeat) if the throttle is pushed open.
One of the behaviors very common with the 4.7 V8 (and blamed on the TPS) is varying engine revs (by about 200rpm) when holding a constant throttle position. Engine surges 200 rpm and returns to normal cruising rpm. It can do this 1-2 times per minute, cruising along at highway speeds. … it will go along at 2000 RPMs at 70…surge to 2200 RPMs for a moment… return to 2000… etc
My engine had been doing that. Now, it doesn’t do it at all. I can’t make it happen since this most recent problem where it bucks under very light throttle/load and throws the CEL on.
Can’t it force an misfire reading? Engine humming along… power cuts out (thanks to TPS)… engine comp picks up misfire… CEL comes on… engine goes into limp mode.
I never said it would. I said the bat TPS would cause a hesitation at that specific throttle opening. If the computer receives a faulty signal, the motor will not run properly. At any other throttle setting, the motor would run normally. The fact that the car will not accelerate is due to the engine going into ‘limp mode’ caused by the computer receiving no signal/signal out of range from the TPS.
The fact that the OP says it always happens under nearly exaclty the same conditions(speed, throttle setting), makes me most suspicious of the TPS.
Exactly.
And even if the engine does not go into limp mode, a tiny incease or decrease of the throttle setting from the ‘bad spot’ will make the run perfectly.