O/T Need Ideas on PU

I have a 87 GMC plow truck. This thing wont run at mid throttle. It idles fine, and will run wide open, but nothing in between. Real hard to plow snow that way. It has a throttle body for fuel injection. It will pop back through the TB at mid range, and if you hold it there it will stall. Let up rael quick and it will idle, or pop it to the floor and it will rev way up. Here is what I have done chasing it so far. Changed the chip in the computer, installed new plugs, new cap, rotor, and module, new throttle position sensor, new idle air control, and retimed it. I haven't changed the fuel filter yet, as it is rusted so bad it I will have to cut out a section of fuel line, and replace it with hose. It was changed about 6000 miles ago when I had the heads redone. I also haven't changed the EGR valve. Any ideas on this pain in back side? Thanks, Jack

Have you "read" the trouble codes?

Change the fuel filter, already, for Pete's sake!

If it was changed 6000 miles ago it's hard to believe it's so rusted you can't get it off with "loosen juice" and some wrenching.

WHY did you chip it...in my experience that seldom makes things better in those early TBI pickups.

Have you tried disconnecting the EGR vacuum line and plugging it (the line)? (The EGR MUST be closing at idle or it wouldn't idle ((same as a BIG vacuum leak)) and disconnecting it will keep it from opening and sticking at mid-range, making the engine stumble.)

Outside of the obvious emissions problems (if that's a worry where you are), driving it with the EGR disconnected will make it a little more prone to detonation under acceleration.

Which brings up another point... if the engine is worn and has valvetrain or timing chain noise it will be picked up by the Electric Spark Control Sensor and interpreted by the electronics as detonation, resulted in undesired timing retard, which can lead to a "stumble".

The test for this is to disconnect the wire from the knock sensor screwed into the RH block drain hole just below and ahead of the starter.

(Remember, with the sensor disconnected the 'puter won't "hear" any REAL knock and retard the timing when that IS needed.)

Change the fuel filter.
No better, disconnect the EGR.
No better, disconnect the knock sensor. (just for testing)

Post back when you've tried these things and let us know if anything helped.

Have you changed out the gasket between the TBI and manifold? That's about the only part of the TBI system that ever fails, and it's only a few bucks.

What about the plug wires?

Sounds like you're sucking air somewhere. May want to log onto a specific forum for that truck. Lot higher chance of finding someone that has experienced/fixed the problem.

Good luck,


Dave

Dave2 and MarkB.. Yes, the TBI base gasket fails, but causes a FAST rough idle. He says his idles good, but doesn't run right at midrange.

I haven't checked the trouble codes as no check engine light is on. Fuel filter is rusted big time as there are no shoulders left on the line nuts, truck gets most of its use in winter, and we have the salt. 6000 miles ago I bought a used engine, was supposed to be a Jasper with 30000 miles on it. It had the older heads on it with the valve seats ground too deep, and I got the newer styled heads, had the valves done. I don't have to worry about emissions here in Mi. I will try as you suggested tomorrow, and let you know what happens. Thanks, Jack

Those rusted fuel lines may have you chasin gleaks all the way back to the sending unit ! . I have had good luck taking air hammer with blunt tip to the fitting on the fuel filter .Give each end a good rattle spray down with PB Blaster , then give them another shot with the air hammer . Surprising how many of those you can get to come loose that way . If not thay make repair fittings you can splice in ..but usually end up all the way up to engine before you find good line to tie into . EGR opening too much too soon will give you fits , unhook line to it and try it . Other than that Fuel pressure and volume are one of the biggest problems with these

I forgot to say I put a electric fuel pump on it this past Spring, because it acted like it was starving for fuel, which it fixed. I'll have to wait until tomorrow to get some fuel line hose. Then I can change the filter, and check the other stuff old. Thanks for more Ideas.

Sound like ignition. Start with the crank sensor. The plastic rots after twenty years, give or take.

There's one I didn't think of, Thank you.

Check the pick up coil in the distibutor.


Russ