Overheat Now Misfiring, Won't Idle

[ian65 0]

Hi Guys,

 

this is my 1992 1.9 GTi...

 

 

 

 

I've owned it for 3 months and in that time I've used it every day and it's run like a Swiss watch.......it had a top end engine build at Skip Browns 3 years ago and has only done about 3000 miles since...... but last Thursday it split a hose on the way to work and overheated.

I replaced the hose, refilled the cooling system but then the car wouldn't idle or drive without missing massively and backfiring and the K light was illuminated.

My initial thoughts were that the steam from the split hose had got into the dizzy cap so I WD'd that to no effect.

The car would fire up on the key fine but after 4-5 seconds it started to miss and then stalled. I ended up getting it recovered back home.

Since then I've pulled off all of the sensor plugs and injector plugs and sprayed them with WD40 to remove any moisture, likewise the HT leads but it's still the same.

 

My next thought was that maybe it'd taken the head gasket although I had stopped the car as soon as the STOP and overheat lights came on.

I compression tested the cylinders and got good even compression across all 4 pots. There is no steam/water from the exhaust, none in the oil, the breathers are clean and the coolant in the header tank seems ok too and the hoses don't seem to be pressurising..... so none of the usual tell tale signs of a blown head.

 

The spark plugs, which were fine last week are very sooty now and it seems to me that the engine is starting up and then after a few seconds, is starting to overfuel which is causing the misfire / stumble / backfire.

 

The car is a Motronic equipped car that has been de-cat'd and unfortunately someone has 'tidied' up the loom in the engine bay and the ECU diagnostic plug has disappeared

 

I've got a few queries that I could do with some help with please.

 

Does anyone know which wire out of the ECU I need to pick up to re-instate the diagnostic plug?

As the K light is on and it seems to be over fuelling I'm thinking that it might be a sensor gone bad..... how likely is it that a overheat would damage say the ECU temperature sensor or something else?

If it turns out to be the head, would a misfire caused by a blown head gasket cause the ECU to illuminate the K light?

 

Anyone got any thoughts that might help me out please.

It seems to be an engine management issue with the light being on but without the diagnostic plug I'm struggling to suss it out.

 

thanks,

Ian

 

 

[pugdamo 111 2 Cars]

Iv only got a Haynes manual to hand but it shows pin 22 from the ECU to the diag socket, iv not had any dealings with the motronic 205's but I think this is the system that you fit an Led in and count the flashes, I'm sure someone will have autodata or something that can confirm for you. Also im not sure what pp2000 can read live data wise from a diag plug? I'm away from my computer with autodata/planet on for the week so I can't check. As you said though I would put it down to a duff sensor if the plugs are black, most probably Ecu temp sensor if it got hot.

 

Nice looking 205 though, very clean.

[ian65 0]

Changed the ECU temperature sensor last night and no improvement... in fact it seems slightly worse.

I have discovered though that if I pull the HT lead off no.4 cylinder there's no change in engine behaviour so maybe I've narrowed it down a bit....... I'll comp test it again and see where I go from there.

[mhyphenl 10 1 Cars]

If it has been de-cat'd what has been done with the Lambda Sensor? Really the ECU makes most of it's decisions based on Temp sensor, AFM and Lambda. The connections for the Lambda tend to be under where the Header tank is so water could have shorted these connections. The K light can be on with head gasket issues but it normally comes on after a minute or so and probably goes off occasionally with certain load conditions. Basically if the light comes on and stays on at start up there is a component issue, ECU thinks its missing or shorted. If the light comes on after a while or occasionally then it's normally something out of range or inability to bring it back into range, or possibly an intermittent fault.

[welshpug 1,666]

number 4 is most common cylinder for hg to fail on.

[ian65 0]

I'm reasonably happy that this isn't a head gasket issue now but more an engine management one.

The car starts on the key and idles great for about 4-5 seconds then starts to miss and eventually stalls. The spark plugs are very sooty as is the tailpipe so it's overfuelling just after the initial start up.

I replaced the ecu temp sensor but it made no difference and eventually I've arrived at the throttle position switch as the possible cause.

I've discovered that if I unplug it the car starts and idles fine but if I plug it back in the above issues return straight away, in fact as soon as I plug it in the car starts to overfuel and misfires then stalls.

I can hear the TPS click as soon as I turn start to open the throttle and I've also had a multimeter on it and it seems to check out ok.

How likely is it that the TPS can click on the opening of the throttle and check out with a multimeter and yet still be faulty and send bad signals to the ECU?

As it stands now with the TPS unplugged, it starts and idles fine but misses a bit when up on the revs as if it's beginning to leaning out which I would expect it to do if the ECU isn't receiving a TPS signal.

 

Any thoughts please guys?

Edited December 26, 2015 by ian65

[mhyphenl 10 1 Cars]

You probably need to ensure you have continuity to the ecu from the tps switch. The switch may work but if there's something amiss in the loom you'll still have issues. I've had too much booze over the last few days to work out what combinations could cause your symptoms. Does seem odd that unplugging resolves the issue for you. Somehow it thinks it's open when it's closed, unplugging effectively is giving a closed. Is there a fully open signal too ?!? Maybe it's seeing this while plugged in?