Engine cuts when idle.

[James85 0]

Hey folks, just after a bit of advice with a little issue I have.the car seems to cut out, sometimes runs a while after starting before cutting , other times does it within seconds. after cuting out it takes a few turns to get it going again, usually have to pump the accelerator a few times too to get it going too, then the revs fluctuate a bit . my next step is to test the fuel pressure to rule out pump and regulator, just waiting for a pressure guage kit to arrive before I can start. Works completed so far, ignition leads changed, plugs changed, distributor and ignition module tested ,  afm rebuilt , new battery. Checked for vacuum leaks. Any advice and insight would be helpful. TIA * I have a video but file is too big to upload 

[Anthony 1,003]

When you start the engine, if you keep your foot gently on the accelerator pedal to keep the revs around 1500rpm or so, will it keep running smoothly?

 

If so, and it's purely an issue at idle, then it's been fairly well documented on this forum over the years for Jetronic (non-CAT) GTI models.  Doing a search will reveal lots of information, but typically it's a setup issue with the throttle body and AFM idle fuel mixture, often not helped by years worth of oily grime inside the throttle body and idle bypass.  Clean the throttle body, follow the Haynes manual for a reasonable starting point with the throttle body, set the hot idle speed around 1000rpm with no electrical load, and aim for around 1.0% CO with the AFM mixture adjustment - this should get you in the right ballpark as long as everything else is in good order.

 

That said, it's also fair to say that you're never going to get a cold start and idle on par with a modern car - when these were new there were negative remarks made in period reviews about cold start/idle, and 30+ years of wear and neglect won't have done anything to improve matters.  Some cars are better than others, but typically, you'll find that they should start on the button from cold, need a second or two of accelerator assistance to keep it running (particularly in colder weather), and then idle by itself.

[James85 0]

Cheers Anthony, was checking a few things over today, I opened up the idle control screw as I thought the revs were sitting a bit low around 1000rpm so it’s now sitting around 1300-1400rpm, stayed in longer after this but did eventually cut out; I tried gently holding accelerator too, again didn’t cut out but felt like it wanted to at one point, had to push the accelerator further to keep it running, I disconnected the TPS also and it didn’t seem to cut out after this, so I’m guessing that’s proven it to be incorrectly set up or faulty? Thanks for the reply 

Edited August 16, 2023 by James85

[Anthony 1,003]

The TPS on these isn't a true throttle position sensor, but rather a fairly crude switch for WOT and throttle fully closed (the latter used by the ECU to activate the overrun fuel cut).

 

It's certainly not unheard of for them to fail, but equally plenty are incorrectly setup - the position should be set so that the switch clicks closed just as the throttle closes and clicks off again just as it opens.

 

Easy to test with a multimeter - it's a three pin connector on the TPS.  I can't remember off the top of my head which is which now, one pin is common, and you should get a circuit with one of the other pins with the throttle closed, and a circuit with the other pin near WOT.

[James85 0]

Thanks for the advice, gonna read up on setting it properly, just incase I do have a spare TPS I could change over with , was planning on changing it when I don’t my accelerator cable as that seems a bit loose too. Will do what you advised with testing it tho first, also setting it.
 

Just out of curiosity, could you run a car without a tps fitted, like I said in previous message the car seems to run fine without it even connected which I thought was odd

[Anthony 1,003]

Engine will run fine without the TPS for testing purposes, but it's not ideal in the long term - specifically, it will feel a bit flat at WOT and you'll likely see slightly worse fuel economy from the overrun fuel-cut not operating.

[James85 0]

Thanks mate, I think back to square one tho, had it running pretty sweet, everything was smooth, since it warmed up tho it was wanting to cut out again having to press the accelerator to keep it going , if I didn’t would just cut out, idle rpm started fluctuating when it warmed up too, was pretty steady when cold 

[Anthony 1,003]

Hunting at idle when warm can be a sign of running a bit rich - try turning the mixture screw on the AFM a couple of turns anti-clockwise to lean the mixture a little and see if it improves things at all.

 

Problem is (as with many issues) that there's lots of things that can cause or contribute to the symptoms that you're seeing, and it's normally a case of working through and making sure that everything is as it should be and ruling possibly suspects out - this is particularly the case on a new-to-you car that you don't know how much it's been messed about with in the past.

 

While cold start/idle can be a bit problematic, as a general rule warm idle should be pretty good and comparatively easy to get right.

[James85 0]

Thanks mate, I think rather than messing about with mixtures blindly, I’ll get it to the garage and see if they can set it correctly , using the probe up the exhaust etc, thanks for your help; deffo things going in the right direction I think tho

[ScoobyJawa 23]

Mines been pretty good to be fair, but I recently removed the K&N air filter and reverted the air intake back to stock and at next MOT it was pretty rich, so I messed with the settings and got it through.  Not having a gas analyzer tho I was stumbling in the dark so when it came up for its next MOT I had my specialist garage do it and asked them to set up all the throttle position, idle etc to make sure its back as good as poss.  It would have cost me more to buy an analyser than the cost of them doing it, and happy in the knowledge a trusted garage has given it a clean bill of health  Starts and idles pretty well for its age now.

Edited August 24, 2023 by ScoobyJawa