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Old 07-12-06, 04:30 PM   #1
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1991 C4 loses power when hot

I have a 1991 COrvette w/39,000 mi. It is fully stock. Automatic L98 engine. It runs excellent for about the first 30 minutes. Then it begins to get sluggish, ie. requires more throttle to maintain speed and hesitates from a dead stop. No codes have been thrown. Used scan tool which did not pick up anything. Had fuel pump tested and tech. said it was losing pressure. Replaced fuel pump 2X with delco pump and car still acts exactly the same. Any ideas?
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Old 07-12-06, 10:51 PM   #2
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Sounds like a job for Super Steve at Bryner! If anyone can diagnose & repair it, he can!
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Old 07-13-06, 06:50 AM   #3
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You may want to replace the gas cap, you maybe getting vapor lock caused by a bad gas cap
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Old 07-13-06, 10:23 AM   #4
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I'm testing to see if the car actually runs better w/ ac on. It does run cooler w/ ac on as a result of electric cooling fans. Service tech insist that the car was losing fuel pressure and the problem was isolated to the pump. I have a hard time believing that 3 pumps act exactly the same. Also, what about installing a fan switch which kicks on at a lower temp? Does anyone have experience with this. Also, I'm considering switching to an aftermarket high volume fuel pump. Any advise here? I will try the new gas cap. Thanks all. Much appreciated. Ed
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Old 07-13-06, 05:32 PM   #5
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If the pump is losing pressure you may have leak down in your injectors. Put a guage on the fuel rail, start the car. See what pressure you're at, turn car off and see if the pressure goes down. If it goes down quickly you may have injector troubles.
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Old 07-18-07, 03:57 PM   #6
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Cool fuelish symptom

My $0.02...

This really does sound like a fuel recirculation problem.

The gas cap is a quick easy thing to try but I wonder if the return circulation to the gas tank is partially clogged or if the EVAP circuit is always shut causing excessive backpressure which would result in the pump looking bad... the problem being that it is fighting against itself.

Problems like this is why Pontiac (no cut against Chevy... they simply have a differnet set up) programmed a short 3 cycle EVAP "test" that runs intermittently (and seemingly randomly) in the LS2 equipped GTO. Us GTO owners have nicknamed that test our "trunk monkey" because when this EVAP test runs, a solenoid fires three times and it sounds like someone/something is in the trunk and is rapping to get let out! As you can imagine, the 1st time I heard this I asked around and found out a bit of why my (now gone) GTO was programmed to do that EVAP test.

Bottom line, this really sounds like a fuel problem but I too doubt it is the pump...
1) gas cap
2) kinked return lines
3) EVAP circuit (fuel fumes need to escape occasionally... same reason why the gas cap is a good thing to try) should be checked.

If the gas cap doesn't help, try this... as you drive and notice the problem, remove the gas cap for a minute, letting the tank pressure equalize.
Get back in and drive... if the symptom is reduced, chances are the EVAP circuit is clogged. Otherwise, you may be looking at partially kinked fuel lines (or a gremlin in your fuel filter )

Let us know, please... this is curious

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Old 07-19-07, 11:53 PM   #7
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I had the same symptom in an LT1 powered '95 Buick I had. Pump was fine, problem was a fuel pressure regulator (located in the engine compartment)
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Old 07-20-07, 08:46 AM   #8
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loss of power

Has anyone ever checked back pressure to see if cat convertor is starting to clog?
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Old 07-20-07, 03:30 PM   #9
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That's what happened to my 90. The EGR valve malfunctioned. The check engine light never came on. It dump unburnt gas into the pre cts. The pre cats desintergrated and clogged the main cat. At low speed the car ran fine. At high speed the back pressure bogged it down and it was always runnig hot of course. I removed the pre-cats and replace the main cat with a high performace cat. The car ran like a top until one fateful day in Ardmore. glug glug.
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Old 01-28-08, 11:51 AM   #10
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I had a similar problem with my 89. The pulsation dampener that connects the fuel pump to the fuel line was bad. I replaced the dampener with in tank fuel line and two clamps and now have 40 psi at the fuel gauge. Not sure if your year has the same dampener.
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Old 12-02-08, 09:55 PM   #11
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The later l98 injectors are very problamatic, it seems darn near every driveability problem we see with the 89+ cars can be attributed to the injectors.
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1991, hot, loses, power

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