Novausa1

I would like to offer my problem to the brain trust that is here at the forum. I have a 1988 Chevy C1500, 4.3 engine, automatic transmission, 2 wheel drive. The truck is basically stock except for the 10.31 LT tires and I have never had the speedometer recalibrated. It is off by about 7 at 60 mph and for every 10 miles showing I have gone 11. I replaced the engine at about 311,000 miles and currently show about 350,000 on the odometer. The engine is mechanically sound, no leaks no knocks and firing on all 6, most of the time. My issue started with a sluggishness that is best explained as the same feel you get when your accelerator pump goes bad. Don’t call me an idiot yet, I know it has TBI, but it had that same dead spot you feel on acceleration. My initial thought was the fuel pump. I had replaced it about 1997 and it tested okay now but with no codes set and that much age I didn’t want to take a chance. So new pump, new sock, new filter and I still had the same problem. The timing was checked and found to be correct and there doesn’t seem to be any slop in the chain. The knock sensor was replaced a few years ago and the timing retards when you tap the block with a hammer. It has received a fresh tune up (wires, plugs, cap and rotor, PCV valve and O2 sensor with all quality parts and still had the same problem. I decided to overhaul the TBI and replace the injectors because it was much cheaper than a remanufactured TBI and the TBI had not been touched internally since I bought the truck and it had 90,000 miles on it. There was a little play in the throttle shaft but not excessive. The TPS was also replaced a while back and currently tests okay so I left it alone. The pintle on the IAC Valve showed some wear so it was replaced and put back together. It seemed to be better, but not fixed; it still had that silly dead spot on acceleration. I still drove it every day but pampered it a little more. After spending a week vacationing at the lake I came back home and the truck acted like it was possessed. It had sat for a week and although it would fire right up would not idle properly and struggled to run when driven. I finally got it to set a code 44 and although it had been replaced and all the tests were passed, I replaced the O2 sensor again. There was no change. I let it set for a couple of days and drove my Yukon instead. That weekend I started it up and it ran fine. It started after a couple of revolutions and idled like it should. I thought the gremlins were gone until I tried to drive it. They were back with a vengeance. I have tested, probed, replaced, cussed and thrown a couple of wrenches but I will not give up. The fuel pressure, on my new fuel pump, is 9-10 # and clamping the return line yields 20#. Vacuum is about 20+ inches at idle, but it will only idle briefly before trying to die. The MAP Sensor, TPS, IAC, both injectors, ECM, PROM (I have a stock and Hypertech) cap, rotor, plugs, wires, ignition module, temperature sensor (for ECM), oil pressure switch (for ECM) have all been replaced. Anything that has not been replaced has been tested and passed except for the Spark Control Module. When cold, the truck starts after cranking for about 3 seconds and begins to idle up and immediately starves for fuel, eventually cutting off. It will restart and goes through the same problem. I can operate the throttle and it will run and rev but will not idle. I can dribble carburetor cleaner in the TBI and it will run and idle like it should. I can unplug the MAP sensor and it will idle on its own. I’ve got nothing else. I’ve hooked my Actron Code Reader to the ALDL and watched for close loop, watched all the voltages from the O2, TPS, IAC and they all are in limits. Both temperature sensors, gauge and ECM work properly. I have capped all vacuum lines, including the Transmission Modulator, which is working properly and the condition does not change. Thus far the only noticeable change was with the MAP Sensor. When it was replaced I was still driving the truck and the downshifts were better. I keep thinking it is the injectors but at this point I question my ability to change the oil…. Just kidding.<O</O

Doug Roberts

Well with all that was done best guess I would make
BAD GAS !!! I seen a lot of bad gas from water to mixtures try dry gas but I would dump it use it for mower or something you don't see any mechanical problems there may not be one

Novausa1

Thanks Doug, when I replaced the fuel pump I wiped out the tank and put new gas in it.

Doug Roberts

Hmmm let me ponder over this a min then was waiting to see if you clean tank

Doug Roberts

OK after readying again thinking a few things fast to check
1 fuel filter I know you prob changed it but see if its on backwards maybe a flow direction
And on the rotor look at the bottem there's a metal tab on some that can fall off if not irk I have a friend I pass this too

Doug Roberts

OK told my friend he says samething .
Do a tune up and recheck everything so I am lost if not anything I said sorry without being there best I can do

Doug Roberts

Any update

Novausa1

Nothing good to report yet. I think I am going to replace the intake gasket this weekend. It almost acts like a vacuum leak but I have been in denial because of the 20+"hg when I start it up. It may be a couple of hour project but at least I can eliminate that from the list of possibilities.

Novausa1

I changed my mind about replacing the gasket. I stoped by my local Goodyear for some new shoes on my Yukon and talked to one of the techs there. He recently had similar problems with an 88 K 1500 and wound up putting a hose clamp on the return line so the fuel pressure would increase. I trid it and it didn't work, but I unplugged my MAP sensor and hooked up my vacuum guage and it idled fairly good. Once it reached operating temprature I plugged the sensor back up and it started and idled great. I didnt take it for a drive but I will do that this weekend.

Doug Roberts

So u need a map??