Warm start problem
10-10-2008, 04:30 PM
#1
AudiWorld Senior Member
Thread Starter
Warm start problem
My daughter's 1995 90 12V has a very strange problem.
When dead cold the car starts and runs perfectly. Once the car has warmed up, if you shut it down the car might start and run rough and then stall or it will not start at all. The only way to get it restarted is to let it sit for a few hours.
I used the VAG-COM a and the coolant temp sensor is reporting the correct temperature (80-100C after shut down).
I am beginning to suspect the fuel pump because but I do not see how that would be affected by temperature.
On this car, when I first turn on the key shouldn't I hear the fuel pump run for about 2-3 seconds to pressurize the system? Then it shuts down until the engine starts. Is this correct?
When dead cold the car starts and runs perfectly. Once the car has warmed up, if you shut it down the car might start and run rough and then stall or it will not start at all. The only way to get it restarted is to let it sit for a few hours.
I used the VAG-COM a and the coolant temp sensor is reporting the correct temperature (80-100C after shut down).
I am beginning to suspect the fuel pump because but I do not see how that would be affected by temperature.
On this car, when I first turn on the key shouldn't I hear the fuel pump run for about 2-3 seconds to pressurize the system? Then it shuts down until the engine starts. Is this correct?
10-11-2008, 07:56 AM
#2
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which coolant switch did you reference... There's 2, the "MFTS" which gives a signal output for the
gauge and AutoCheck..it's usually a 3-4pin sender. Then there's a 2pin white one which is used by the ECU for cold-starts and fuel metering. I'm not positive which sensor value VAG-com would be referencing during a scan...for that info you'd need to refer to a Bentley.
I had a near identical issue as your's but with my 10v 90. Started up beautifully and ran well when cold, but after warming up it would never start. The NG is pretty radically different from an AFC but I think similar issues could have similar fixes. Initially I was able to make the motor start/run (when hot) by manually closing the WOT enrichment switch. I don't know why this fixed the problem but it did. The next thing I replaced was my 2pin (black) ECU coolant temperature sender (G62). When I first was testing it, none of the resistance values matched up vs. their associated coolant temps. I thought for certain I'd found the problem, and it made sense that if the ECU was totally srewball with what temp it thought the motor was it would subsequently over fuel. So I bought a new sender ($100+ yikes), put it in, ran the car, compared values, and I was coming out with almost the same sort of numbers as with the old sender. BUT, after that bit of tinkering the car starts and runs hot...so unfortunately in the end I don't really know what fixed the problem; but those were the pieces I was poking at.
You might consider warming up the car (90C, first stage cooling) and then putting a multi-meter on that white 2pin sensor. Just see what the Ohms are and there is a reference table in the OBD section of Bentley.
I personally don't think it's the fuel pump, because if that were the case why does the car start & run properly from cold. If the pump were FUBAR then the engine would be cranky all the time.
Yes, on initial key-start the pump will run, but sometimes is VERY difficult to hear it from the front seat. You'd be better to educate your daughter by having her in the trunk with her ear against the fuel tank access plate listening for the buzz.
Other than all that unfortunately it's a tricky little gremlin.
I had a near identical issue as your's but with my 10v 90. Started up beautifully and ran well when cold, but after warming up it would never start. The NG is pretty radically different from an AFC but I think similar issues could have similar fixes. Initially I was able to make the motor start/run (when hot) by manually closing the WOT enrichment switch. I don't know why this fixed the problem but it did. The next thing I replaced was my 2pin (black) ECU coolant temperature sender (G62). When I first was testing it, none of the resistance values matched up vs. their associated coolant temps. I thought for certain I'd found the problem, and it made sense that if the ECU was totally srewball with what temp it thought the motor was it would subsequently over fuel. So I bought a new sender ($100+ yikes), put it in, ran the car, compared values, and I was coming out with almost the same sort of numbers as with the old sender. BUT, after that bit of tinkering the car starts and runs hot...so unfortunately in the end I don't really know what fixed the problem; but those were the pieces I was poking at.
You might consider warming up the car (90C, first stage cooling) and then putting a multi-meter on that white 2pin sensor. Just see what the Ohms are and there is a reference table in the OBD section of Bentley.
I personally don't think it's the fuel pump, because if that were the case why does the car start & run properly from cold. If the pump were FUBAR then the engine would be cranky all the time.
Yes, on initial key-start the pump will run, but sometimes is VERY difficult to hear it from the front seat. You'd be better to educate your daughter by having her in the trunk with her ear against the fuel tank access plate listening for the buzz.
Other than all that unfortunately it's a tricky little gremlin.
10-11-2008, 04:20 PM
#3
AudiWorld Senior Member
Thread Starter
I found the problem - Fuel Pump
Quattro.pilot, thanks for the advice. Figures it would be someone who is also on the S-Car list. That list is the best when it comes to repair advice.
From the Bentley and from a little logic I was certain I was reading the correct temp sensor with VAG-COM. After all, VAG-COM talks to the ECU when you use the Engine Block so it must be querying the same sensor the ECU uses.
So next I checked the crank position sensor but it was OK. Also Cam and Crank position sensors, while only need at startup could go bad when hot but they both throw codes and there were no fault codes.
Next I pulled a plug and checked for spark. Sure enough, there was spark so the ignition and probably the injectors were running fine.
OK, now it was down to the fuel pump or fuel pump relay. Trying to avoid the pump job I checked the relay first. Connected it to 12V with an ohm meter across the pump power pins and it closed perfectly.
Finally I checked the pump. I disconnected the return line from the injector fuel rail, no pressure. I then shorted the relay fuel pump pins - no gas.
So I dove into the tank (after siphoning about 8 gallons since the car was almost full) and removed the pump. Fortunately, thanks you the S-Car forum I purchased a fuel pump a couple of years ago just in case after hearing it was a common failure item. Both cars use the same pump so I installed it and the car started right up.
By the way, the cold start theory died once the car was in the garage. I let it sit overnight and it still would not start. I think the pump was going a little at a time and perhaps when it was cooler the pump could get itself started.
I wanted to post this to add it to the knowledge base in the event someone else runs into the same problem.
From the Bentley and from a little logic I was certain I was reading the correct temp sensor with VAG-COM. After all, VAG-COM talks to the ECU when you use the Engine Block so it must be querying the same sensor the ECU uses.
So next I checked the crank position sensor but it was OK. Also Cam and Crank position sensors, while only need at startup could go bad when hot but they both throw codes and there were no fault codes.
Next I pulled a plug and checked for spark. Sure enough, there was spark so the ignition and probably the injectors were running fine.
OK, now it was down to the fuel pump or fuel pump relay. Trying to avoid the pump job I checked the relay first. Connected it to 12V with an ohm meter across the pump power pins and it closed perfectly.
Finally I checked the pump. I disconnected the return line from the injector fuel rail, no pressure. I then shorted the relay fuel pump pins - no gas.
So I dove into the tank (after siphoning about 8 gallons since the car was almost full) and removed the pump. Fortunately, thanks you the S-Car forum I purchased a fuel pump a couple of years ago just in case after hearing it was a common failure item. Both cars use the same pump so I installed it and the car started right up.
By the way, the cold start theory died once the car was in the garage. I let it sit overnight and it still would not start. I think the pump was going a little at a time and perhaps when it was cooler the pump could get itself started.
I wanted to post this to add it to the knowledge base in the event someone else runs into the same problem.
10-12-2008, 08:26 AM
#5
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cool!! as much as it sucks that i was kinda wrong, i'm glad you figured the problem. well done
siphoning is not fun, heheh =(
BTW, just a matter of curiosity, did you break the return line after the fuel pressure regulator or before it?
BTW, just a matter of curiosity, did you break the return line after the fuel pressure regulator or before it?
10-12-2008, 08:59 AM
#6
AudiWorld Senior Member
Thread Starter
Actually it was the supply line
I broke it right on top of the manifold where it turns from solid line to flex. Actually, after checking the Bentley for the 12V it was the supply line that I broke so it was before the FPR.
The Bentley drawing I was working from before was not so clear. Your question made me go back and find a better illustration.
The Bentley drawing I was working from before was not so clear. Your question made me go back and find a better illustration.
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10-12-2008, 01:11 PM
#8
AudiWorld Senior Member
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Being a father I will not post pictures but...
She is a photography, graphic designer. She is the one who did the illustration in my signature.
She is an Audi lover also.
She is an Audi lover also.
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