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Stumble at Idle

danmanTSI

By danmanTSI,
in Flatbody Mechanic

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danmanTSI Newbie

danmanTSI

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Since I got my 84 Its had a very present stumble at idle. So far I've replaced the cap/rotor, spark plug wires, new ngk plugs gaped to .36, deleted egr valve, got new lower shores 850cc injectors to replace the leaky stock ones, and fixed a couple minor vacuum leaks coming off the tb ports. Since then I've also went around the entire car with a propane torch trying to find additional leaks, but couldn't find any. compression is an even 130 across all four cylinders. Like I said the stumble is very present and causes the car to shake slightly at idle. Please help, I'm gonna go crazy!!

 

Dan

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alp247 Newbie

alp247

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Just wondering if you pulled any of the error codes. It could be you need to do the TPS reset but everyone is going to give alot of things to randomly check but i would try to pull the codes and then check those areas that the code is relating to. it could be you have a leak at the from the boot to the turbo as well.
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Metric-man Newbie

Metric-man

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Your ISC/TPS is probably not original, and your fuel rail and or throttle body might be borrowed from a latter model vehicle.

Another reason contributing to this is aftermarket injectors have a different spray pattern and fuel puddles inside the throttle body and intake manifold.

Have you checked the injector clips?

Do you have a BOV?

 

 

It could be because Pre 85 did not have Cold start/High altitude compensation; at idle you’re going to notice the ECU checking the sensors about every 7 to 10 seconds.

The TBI injection is basically a feedback carburetor and it is important to have the TPS and ISC adjusted correctly and depending how well it is adjusted you’re going to notice it a lot or a little.

It is not as critical in 87 and later models because of the nose switch at the tip of the ISC helps idle control.

The intake manifold has a ground wire; yours might need replacing or contact points cleaned.

 

 

Other reasons could be the distributor base oring an oil seal leaking blow by into your cap or internal igniter might be on its way out.

The throttle body may need to be over hauled with the base and lemon shaped oring gasket ; you might have a slight coolant leak and it could be contaminating your o2 sensor.

 

What is your curb idle speed?

Is it the same cold as it is hot with the upper radiator hose expanded?

 

What is your upper radiator hose coolant thermostat rated at 160, 180, or 190?

Is it working?

 

Do you have an exhaust leak around your exhaust manifold or gasket?

Are all the studs there, and not loose?

Edited by Metric-man
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MikeMeyerhoff Newbie

MikeMeyerhoff

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this is happening all the time correct? (not just cold start?)

 

Try to keep the main coil plug wire, alternator charge + feed wire, and ignitor (and harness) reasonablly away from eachother. There is a chance of interference that can cause stumbling/misfire issues.

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danmanTSI Newbie

danmanTSI

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Okay, I've been very busy with work and school lately. I did get a chance to check the idle speed though, cold idle is 1900 and eventually eases itself down to a constant 850-900 rpms. As far as I know the car has the stock thermostat (yes I know it should be replaced) and it works perfectly. The problem existed before I bought the new injectors and the rest of the fuel system is bone stock as is the entire car. There are no exhaust leaks, and the manifold along with studs are perfect. The car currently has 35,000 miles. Yes the problem exists when the car is both cold and warm. I'm off tomorrow and will have some more time to check out the other suggestions.

 

Thanks,

Dan

Edited by danmanTSI
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Caliber308 Collaborator

Caliber308

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Just wondering if you pulled any of the error codes. It could be you need to do the TPS reset but everyone is going to give alot of things to randomly check but i would try to pull the codes and then check those areas that the code is relating to. it could be you have a leak at the from the boot to the turbo as well.

 

Excellent advice.

 

Bill

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Metric-man Newbie

Metric-man

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There will be a two prong female connector in the engine compartment on the right side just left if the inner fender.

 

http://www.starquestclub.com/forum/index.php?showtopic=28043&hl=codes&fromsearch=1

 

If the rest of your fuel system has not been serviced even though the vehicle has low milage

you're going to need to address that independent of researching the ECU error code sequence.

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alp247 Newbie

alp247

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keep us posted on what you find. I'll do my best to check in often and give some advice. Try to get those codes and lets go from there. it's best to hook up a light or a led that can can handle 12v. You can use a multimeter but if you have a bulb or led attached to it all the time you can check if you ever get any code errros when your car starts acting up.

 

Everyday when i start my car i just pop open the glove box and check if i have any codes from the light blinking. I always get the single blink for the O2 sensor (normal) and then it stays on when it's warmed up.

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  • 3 months later...
danmanTSI Newbie

danmanTSI

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I know its been a while since I've last updated this thread but I did manage to narrow things down a bit. All the vacuum lines have since been replaced with new and I also did a tps/isc reset. (with no change) Though I did fine something out, when the a/c compressor is on the idle remains unchanged, but the stumbling goes away. So my next question is what changes with the A/C on?
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ZacMan Newbie

ZacMan

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Does yours have the 'ECI' cap injection system, or the t-bar style fuel rail?

 

Turning on the AC richens up the mixture a little and should up the idle slightly. The method of upping the idle is opening an extra air pathway on the ECI cap models, and advancing the stepper motor slightly on the t-bar models (I believe, don't quote me on that one, I don't own one).

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MikeMeyerhoff Newbie

MikeMeyerhoff

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The only thing that should change with the AC running is the idle point.

 

Its sounding like your tps/isc may be off somewhat. They are VERY tricky on the early cars because there is no feedback from a motor position sensor like the later cars have. The ECU basically adjusts idle by running the stepper motor for a certian amount of time. So if the stepper motor doesn't move as much as the computer expected it to, the idle can suffer.

 

I use a vaccum guage to help set mine. The 3 ports on the tb "see" vaccum at different throttle angles. So watching the guage will help tell you where the throttle plate really is. If I remember correctly, I monitor the center port to ensure that it doesn't see vac at the idle point.

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MikeMeyerhoff Newbie

MikeMeyerhoff

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I should add that other things can affect how much the stepper motor actually moves in a given amount of time. If there is resistance due to wear in the tb itself. Or if the tb was rebuilt and the spring was wound a different amount of times, that can change how fast the stepper motor operates also.
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