1987 Chevy Truck engine very lazy off the line

Tiny
RUMBLEBEE
  • MEMBER
  • 1987 CHEVROLET TRUCK
Engine Performance problem
1987 Chevy Truck V8 Four Wheel Drive Automatic

Okay, here is the problem. I took a TBI unit off an 87 chevy and installed it on an 85 chevy, and it runs pretty good with one exeption, when you hit the throttle from idle the engine really lacks performance to the point of backfiring sometimes, It is very lazy to get going.I can mask the problem by bumping the base timing up to 10 degrees and it runs much better, but still lazy. I just installed a brand new engine yesterday thinking that would solve all the problems, but it made no difference ( the old engine was wore right out ) Fuel pressure is not the problem, It has been checked and rechecked, I installed new injectors too. No fault codes. What I did find is when the engine is accelerated the timing is retarting, alot. The timing retards to after TDC for a short period od time and then will advance up to as high as 24 degrees total. With the base timing bumped up to to 10 degrees I get a total of 34 degrees and it runs much better, but still retards on initial rev up. I checked my buddies truck, and his timing retards much less and much quicker on his truck, and his will actually spin the tires. HELP, I am stumped and dont want to just start changing more parts at more expense that dont help, and dont really want to put an HEI unit in to bypass the problem.
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Saturday, April 5th, 2008 AT 9:32 AM

7 Replies

Tiny
RUMBLEBEE
  • MEMBER
Update: So I played around for a few hours, and I tested the wiring to the ESC, and all is well, there is 9.43 volts to pin b7 on the ECU. I checked the injector pulse and they look good, then I removed the TPS and bent the actuating arm on it to simulate more throttle valve angle, and that made a big improvement. If anyone knows what the reference voltage should be at idle, I would be glad to have it so I can set it to spec.
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Saturday, April 5th, 2008 AT 7:19 PM
Tiny
MASTERTECHTIM
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Tps should be.50 volts at idle. Remove dist cap and look at the mainshaft in the distributor and look for cracks. I have replaced a hundred mainshafts in these distributors to fix various timing problems. How does it run if y ou leave the est connector unplugged? Also you need to check vaqcuum at the map sensor. These have problems with carbon build up under the throttle body, restricting vacuum to map sensor causing it to run lean on hard accel due to lack of vacuum. Get back to us
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Saturday, April 5th, 2008 AT 7:49 PM
Tiny
RUMBLEBEE
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I tried leaving the EST unplugged and it runs the same, I also changed the ESC and MAP sensor today with known good ones and it made no change, I will set the TPS to proper spec tomorrow, but am wondering what the minimum air rpm is? I am guessing somewhere around 475 rpm? This could cause the laziness off idle too maybe? I will clean the throttle body (again ) and set minimum air and tps and hope that works. What causes the timing to retard on initial throttle opening? Should it do this? Also, should the control module advance the timing with the ESC wire unplugged? Mine does not.
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Saturday, April 5th, 2008 AT 8:50 PM
Tiny
RUMBLEBEE
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Update- I bought a new distributor today and it made no improvement to speak of. It does have some built in timing advance, but really does not help the problem. Help!
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Saturday, April 12th, 2008 AT 9:36 PM
Tiny
MASTERTECHTIM
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Have you checked the vacuum to the map sensor?
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Sunday, April 13th, 2008 AT 6:58 AM
Tiny
RUMBLEBEE
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My actual vacuum gauge is broken, but it has a strong signal to it, and it dies instantly if I disconnect it while it is running.
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Sunday, April 13th, 2008 AT 7:56 AM
Tiny
RUMBLEBEE
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Well, it turned out to be a faulty ECM, changed it out tonite, and everything smartened up.
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Wednesday, April 16th, 2008 AT 8:05 PM

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