fuel injection mystery (long)
08-21-2003, 05:16 PM
#1
Instructor
Thread Starter
fuel injection mystery (long)
It's been a long time since I was member here - things have changed!
I was curious if there are any brains out there who can figure out this problem. Car is at the shop and I don;t know the diagnosis yet.
Car: 1985.5 944 race car
Problem description: During a warm-up for a race a few months ago, the car died. It was not really warm, and had been running well the previous day. After a couple of minutes the car would run fine, but quit again after about 20 secs. Doesn't matter if the car revs or idles, still quits after about the same amount of time (meaning the fuel consumption rate is not the issue).
I checked a multitude of things: spark was there when the motor quit, fuel seems to be OK, changed the following: fuel pump, filter, reference sensors, DME relay, DME. It appeared that the problem was in the fuel injectors. We noticed that if you unplug an injector, it would run fine without quitting. Plug the injector in and the motor would die. We took the cover of the injector rail and the car ran great. It appeared a wire may have shorted against a sharp edge on the fuel rail. The extra load of 4 injectors vs. 3 caused the computer to stop firing the injectors. Duct tape fixed it.
I ran that race, a DE, and an SCCA enduro. Next I go to a race in South Carolina. 9:00 AM and everything is fine. i sat on the grid, motor running, for about 20 mins. As they waved us onto the track the car dies. Exactly the same symptoms.
I worked on it all day. I changed the kill switch, ref sensors (again), DME, etc. I also checked the air flow meter and throttle swicth, and tested and bypassed the engine temp sensor. Ground connections were fine. Finally started looking at the injectors closely again. Either there was a bad (shorted) injector, or the wiring harness is bad. I checked the harness, pulled the insulation off and did resistance tests. Everything seemed fine. Connectors are clean, etc. The ground distributions were very clean. The power distribution to the injectors looks fine. The voltage to the injectors looks fine. The injectors show the correct resistance. Still, pull an injector, any injector, and it runs for ever. Plug it in and the motor dies right away. I finally gave up thinking it's a bad injector (when it gets warm) and gave up. When I get home I put in new injectors (Judging by the amount of fuel that came out of the rail fuel pressure is not the problem). The new injectors exhibited the exact same behaviour!
It could still be wiring. I do not have a spare harness. If anybody has any other ideas, take a guess. I'll post the result when it's solved.
Timo
I was curious if there are any brains out there who can figure out this problem. Car is at the shop and I don;t know the diagnosis yet.
Car: 1985.5 944 race car
Problem description: During a warm-up for a race a few months ago, the car died. It was not really warm, and had been running well the previous day. After a couple of minutes the car would run fine, but quit again after about 20 secs. Doesn't matter if the car revs or idles, still quits after about the same amount of time (meaning the fuel consumption rate is not the issue).
I checked a multitude of things: spark was there when the motor quit, fuel seems to be OK, changed the following: fuel pump, filter, reference sensors, DME relay, DME. It appeared that the problem was in the fuel injectors. We noticed that if you unplug an injector, it would run fine without quitting. Plug the injector in and the motor would die. We took the cover of the injector rail and the car ran great. It appeared a wire may have shorted against a sharp edge on the fuel rail. The extra load of 4 injectors vs. 3 caused the computer to stop firing the injectors. Duct tape fixed it.
I ran that race, a DE, and an SCCA enduro. Next I go to a race in South Carolina. 9:00 AM and everything is fine. i sat on the grid, motor running, for about 20 mins. As they waved us onto the track the car dies. Exactly the same symptoms.
I worked on it all day. I changed the kill switch, ref sensors (again), DME, etc. I also checked the air flow meter and throttle swicth, and tested and bypassed the engine temp sensor. Ground connections were fine. Finally started looking at the injectors closely again. Either there was a bad (shorted) injector, or the wiring harness is bad. I checked the harness, pulled the insulation off and did resistance tests. Everything seemed fine. Connectors are clean, etc. The ground distributions were very clean. The power distribution to the injectors looks fine. The voltage to the injectors looks fine. The injectors show the correct resistance. Still, pull an injector, any injector, and it runs for ever. Plug it in and the motor dies right away. I finally gave up thinking it's a bad injector (when it gets warm) and gave up. When I get home I put in new injectors (Judging by the amount of fuel that came out of the rail fuel pressure is not the problem). The new injectors exhibited the exact same behaviour!
It could still be wiring. I do not have a spare harness. If anybody has any other ideas, take a guess. I'll post the result when it's solved.
Timo
08-21-2003, 06:47 PM
#2
Sounds like the fuel pressure regulator:
http://www.clarks-garage.com/shop-manual/fuel-07.htm
From the clark's garage web site:
"For years, 944 owners and even experienced Porsche mechanics, have on a regular basis replaced perfectly good DMEs, fuel pumps, and various other components because of faulty fuel pressure regulators (FPR). However, it's quite understandable as faulty FPRs can display a wide range of symptoms depending on how the regulator fails. Also, the function and location of the FPR is quite often confused with the fuel damper. The fuel damper is on the supply to the fuel rail and functions to "dampen" the pulsations caused by the fuel injectors. The fuel pressure regulator is located on the return to the fuel tank and functions to control the pressure on the fuel rail. The photos below shown typical 944 fuel rail arrangements and the location of the fuel pressure regulator."
"One of the most common failure modes for the FPR is to fail closed to the point that fuel rail pressure is extremely high. The high differential pressure across the injectors causes them to draw excessive current. The excessive current is seen by the injector drivers which subsequently shutdown. If you have a condition where the car will not start or starts and almost immediately dies, try disconnecting the wire for one (1) injector. Attempt to start the engine. If it starts and continues to run, the fuel pressure regulator is likely bad. Bear in mind that running on only three cylinders, the car will run very rough. By disconnecting one injector, it reduces the current enough to keep the injector drivers from shutting down."
http://www.clarks-garage.com/shop-manual/fuel-07.htm
From the clark's garage web site:
"For years, 944 owners and even experienced Porsche mechanics, have on a regular basis replaced perfectly good DMEs, fuel pumps, and various other components because of faulty fuel pressure regulators (FPR). However, it's quite understandable as faulty FPRs can display a wide range of symptoms depending on how the regulator fails. Also, the function and location of the FPR is quite often confused with the fuel damper. The fuel damper is on the supply to the fuel rail and functions to "dampen" the pulsations caused by the fuel injectors. The fuel pressure regulator is located on the return to the fuel tank and functions to control the pressure on the fuel rail. The photos below shown typical 944 fuel rail arrangements and the location of the fuel pressure regulator."
"One of the most common failure modes for the FPR is to fail closed to the point that fuel rail pressure is extremely high. The high differential pressure across the injectors causes them to draw excessive current. The excessive current is seen by the injector drivers which subsequently shutdown. If you have a condition where the car will not start or starts and almost immediately dies, try disconnecting the wire for one (1) injector. Attempt to start the engine. If it starts and continues to run, the fuel pressure regulator is likely bad. Bear in mind that running on only three cylinders, the car will run very rough. By disconnecting one injector, it reduces the current enough to keep the injector drivers from shutting down."
08-21-2003, 06:50 PM
#3
Addict
Rennlist Member
Rennlist Member
My mom's 84 NA did the run for a few and die thing too, also died a few times(very rarely) while driving. Finally, it just wouldn't start one morning. I figured it was the DME all along, I replaced the relay and it's been fine since. It sounds like the only thing left is the harness for you?
08-21-2003, 08:28 PM
#5
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Actually the DME does not shut the injectors down. There just isn't sufficent current to open 4 against the high pressure while there is for 3.
I had the impression that the injectors on the 944's opened in pairs.
The injectors on the 944S do fire four at a time. Guess you have to with two intakes per bucket.
Speaking of injectors: Had this idea to put two injectors per cylinder and fire both at higher rpms.
I had the impression that the injectors on the 944's opened in pairs.
The injectors on the 944S do fire four at a time. Guess you have to with two intakes per bucket.
Speaking of injectors: Had this idea to put two injectors per cylinder and fire both at higher rpms.
08-21-2003, 10:10 PM
#6
Instructor
Thread Starter
All four injectors do fire simultaneously.
Check in the Bosch fuel injection book about a LITTLE info about doubling up injectors. it would be easier to use larger injectors and re-map the DME, but the big problem in the 944 is getting enough air into the intake (at least that's my understanding).
Check in the Bosch fuel injection book about a LITTLE info about doubling up injectors. it would be easier to use larger injectors and re-map the DME, but the big problem in the 944 is getting enough air into the intake (at least that's my understanding).
