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I bought my 1991 S4 back from the USA in 2018 and I've tried all different brands and grades of petrol, but I'm having real issues, where after about 2-3 miles after I've filled up, it starts to miss fire, and sometimes feels like its going to cut out on me. Then after about 10-15 miles it will usually be OK with no issues. I've also tried an octane booster but that made it even worse. I have taken taken it to my mechanic but they can't seem to find the issue. Any input would be appreciated.

Thanks
 

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But Mad North-Northwest
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I doubt it's an issue with the petrol quality assuming you're using whatever the equivalent of premium is there (RON 95 or 98, I assume.)

If it only happens after filling the tank, I would suspect an issue with the tank vapor recovery system. Next time you fill it up leave the gas cap loose so it can vent. If the problem goes away then that isolates the vapor system and you can troubleshoot from there.
 

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Just a guess - is the tank venting system working properly? Seems odd it'd act up so soon after a fill up but...well, it's just a random thought. You could try filling up and then driving a few miles with the gas cap off or loose enough that it is not sealing. If that solves the issue then I'll let you know where to send my fee.
 

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I bought my 1991 S4 back from the USA in 2018 and I've tried all different brands and grades of petrol, but I'm having real issues, where after about 2-3 miles after I've filled up, it starts to miss fire, and sometimes feels like its going to cut out on me. Then after about 10-15 miles it will usually be OK with no issues. I've also tried an octane booster but that made it even worse. I have taken taken it to my mechanic but they can't seem to find the issue. Any input would be appreciated.

Thanks
If it's running fine after 10-15 miles, I doubt it's the petrol. Poor fuel quality will cause pinging, but it won't just go away, and in fact would likely get worse on a warm engine.

A shot in the dark, but perhaps an electrical issue with the ECU? 30 year old electronics can do strange things.
 

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UK petrol will be superior, as far as octane goes, to what you can get in many/most US States...very much doubt it is the petrol, therefore;)

I don't get the 'where after 2-3 miles after I've filled up' thing...you mean it only mis-fires after you have filled up?
could be junk in the tank getting moved about...or a dirty main petrol filter (change that anyway if you don't know the age of it, costs a few quid, takes 15 minutes)

But it sounds more 'electrical' or temperature related, since you say things clear up - 'after about 10-15 miles' - if that is the case then you should check the motronic temperature sender (that's the blue sender near the injectors), one of the more important 'bits' of the ML4.1 motronic system.

motronic temperature sensor.JPG

remove the blue plug and see what ohms you get between both pins, cold engine...mine is about 1400 Ohms
If you need to replace it the PN is
Bosch 0 280 130 026
about a tenner.
 

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UK petrol will be superior, as far as octane goes, to what you can get in many/most US States...very much doubt it is the petrol, therefore;)
Keep in mind that the U.S. and Europe use different ratings for octane. What the U.S. calls "93" octane is equivalent in octane content to 98 in Europe because vendors typically quote RON while the U.S. uses the RM/2 method.


Generally, fuel is pretty equivalent between U.S. and Europe, with the exception of places like California where you can only get 91 octane.
 

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Discussion Starter #7
Keep in mind that the U.S. and Europe use different ratings for octane. What the U.S. calls "93" octane is equivalent in octane content to 98 in Europe because vendors typically quote RON while the U.S. uses the RM/2 method.


Generally, fuel is pretty equivalent between U.S. and Europe, with the exception of places like California where you can only get 91 octane.
Thanks for the info.
 

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Discussion Starter #8
I doubt it's an issue with the petrol quality assuming you're using whatever the equivalent of premium is there (RON 95 or 98, I assume.)

If it only happens after filling the tank, I would suspect an issue with the tank vapor recovery system. Next time you fill it up leave the gas cap loose so it can vent. If the problem goes away then that isolates the vapor system and you can troubleshoot from there.
Thanks, I'll try that first.
 

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Discussion Starter #9
UK petrol will be superior, as far as octane goes, to what you can get in many/most US States...very much doubt it is the petrol, therefore;)

I don't get the 'where after 2-3 miles after I've filled up' thing...you mean it only mis-fires after you have filled up?
could be junk in the tank getting moved about...or a dirty main petrol filter (change that anyway if you don't know the age of it, costs a few quid, takes 15 minutes)

But it sounds more 'electrical' or temperature related, since you say things clear up - 'after about 10-15 miles' - if that is the case then you should check the motronic temperature sender (that's the blue sender near the injectors), one of the more important 'bits' of the ML4.1 motronic system.

View attachment 1623638

remove the blue plug and see what ohms you get between both pins, cold engine...mine is about 1400 Ohms
If you need to replace it the PN is
Bosch 0 280 130 026
about a tenner.
Thank 's for that, i'll test that later.
 

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Stumbles right after filling the tank up then goes away after 10-15 minutes of driving.

Question:
Do you always fill up at the same level of fuel in the tank, like the when the low fuel light comes on?

Checks I would do:
1. Try filling up at different tank levels and see if it does it.
2. Clean all the electrical grounds.

Completely go through the fuel system:
1. Change the fuel filter and drain the inlet side to see if debris or rust color fuel comes out of it.
2. Pull the intank fuel pump out and shine a light in the tank to check for rust. Also check its filter for debris.
3. Check and clean the main fuel pump connections, check pump's ohms, check voltage to the pump and test the fuel pressure out of the pump.
4. Check the check valve in the gas vapor vent system to see that it's working.
 

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Discussion Starter #11
Thanks for you're input. Since I've been back in the UK I've had a new petrol tank (had been repaired a few years ago), and just recently a new pre pump and new fuel pump, this was all done as the car completely died on me one night. I will get the electric's checked out and i'll look at the gas vapor vent system, and I'll start filling up at different levels and maybe not fill the tank.
 

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If you feel like spending a few more quid you could try replacing the Fuel filter with one supplied with an optional pressure gauge (from normal UK suppliers). It's not a bad thing to do anyway and the gauge will tell you if there's a fuel pressure problem near the carbs in a running situation. I think it's normal practice to only use the pressure gauge for setup and once all work is done to replace with a plug.

If your filter already has a port for pressure gauge already then just replace element and only get the gauge. If all is ok up to there then at least you know your fuel supply is good and you can start looking at other causes.
 
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