GM TBI Problem-Need help!

[SpeedMetal]

The wife's Blazer is running rough-it's an 89 with a 5.7 and TBI. Under acceleration, it stumbles and almost dies. However, at constant speed and at idle it runs perfect and has good power. I've replaced the TPS, fuel pressure regulator, fuel filter, cap and rotor. It's also hard to start, sometimes it takes 5 or 6 tries. I'm thinking now that the fuel pump isn't putting out enough pressure. Any and all help is much appreciated.

[matt_helton]

deffinetly could be the fuel pump. those pumps are real cheap even for OEM.

also for kicks, check the timing and make sure the distributor hasnt moved at all. and dont forget to dissable the ESC by disconnecting the tan single wire connector on the pass side of the fire wall.

and how old are the plugs and wires?

[SpeedMetal]

The plugs and wires are fairly new....I'm gonna try replacing the fuel pump tomorrow and hopefully that'll do it. I just hope it gets me home tonight....

[FABRICATOR]

The MAP sensor plays a major role in throttle response.

[SpeedMetal]

Well, the fuel pump didn't fix it. I'm thinking that the MAP sensor will probably be the next step. Would a bad MAP sensor make it hard to start? I barely got it to start today, it took like 10 tries. It's idling and running at constant speeds really good. Just off-idle starts anything past 1/4 throttle are horrible, almost dies and the starting sucks. Thanks again for the help.

[matt_helton]

did you check the timing? how many miles are on this motor? ive seen timing chains become stretched over time and actually skip a tooth on the crank or cam causing problems like you are having.

map sensor, ok......but shouldnt cause hard starting.

id also check the coil output.

could also be an ign. module/pick up coil taking a dump.....which means distributor overhaul.

id also check the egr operation.....make sure its not stuck open.

[Ryno]

a couple other things to check....IAC....vacuum hoses, the pcv valve, and the EGR. My 94 k1500 was real touchy when any of the above was dusted. Distributor is $100 from autozone. A coil is like $30. check them both, I had problems at least 5 times with the coil arcing.

[FABRICATOR]

My bad, I didn't see the hard starting part in the original post. Most everything starts in open loop anyway. Are you getting a good spark while cranking?

[SpeedMetal]

Tried to start it this morning, no go. It only tried to fire for one brief second. I'm thinking I'm gonna try the module now....I'll check out the timing and coil output as well. The motor has about 25,000 miles on it, and runs really good(just not right now).

[matt_helton]

25,000 or 250,000?? didnt you say it was an 89? or is it a second motor for the truck?

when your cranking it can you see the injectors squirting fuel down the TB?

[SpeedMetal]

No go on the module. It's getting fuel, no spark. The motor has maybe 25k on it. It's in good shape, 2nd maybe 3rd motor.

[SpeedMetal]

Swapped out the coil and that did it. Running perfect. However, there's one white wire coming out of one of the plugs that go into the coil that looks like it's just been dangling, unconnected. Any idea where that should go? Thanks again for the help.

[matt_helton]

2 wires from the coil connector should go back into the engine harness and the other 2 should go to the ign. module in the distributor. there shouldnt be any extra wires hanging around.......

[SpeedMetal]

I can't seem to find where that wire should go....it's about 5" long and has what looks to be a factory connector on the end. It comes out of the gray side of the plug that goes into the coil. If it's a ground wire can I just ground it?

[matt_helton]

what kinda of connector is on it? like a loop for a bolt or an actual plastic connector with a seal to connect to another wire or mudule?

[matt_helton]

tkr.......help me out here.

[SpeedMetal]

Here's a pic of the straggler.....

[tkr]

tkr.......help me out here.

Maybe an accessory tach wire? I can look at a diagram tomorrow.

[SpeedMetal]

Maybe. The truck doesn't have a tach in it. Another thing is that I've had the truck for about a year and a half, and it has run great the whole time. And from the looks of that plug, it doesn't look like it's been plugged in for a long time. I'm just wondering if that's why the coil finally went bad. Could it be that the coil should have this extra ground, but can operate for a while without it? TKR, I'd really appreciate it if you could check out a GM diagram and see where that little bastard should be. Mucho gracias guys.

[matt_helton]

yeah, prolly a tach wire. i wouldnt sweat it.

[tkr]

TKR, I'd really appreciate it if you could check out a GM diagram and see where that little bastard should be. Mucho gracias guys.

Just checked alldata and there is a white wire there for a tach. Since your truck doesn't have one, that makes sense. Nothing to worry about. Glad you figured out the problem.

[FullsizeFun]

So it was the coil then. Thats good to know cuz my truck just started doing the same thing (92 GMC). Idles fine, drives on fwy etc fine, but stumbles when taking off from a stop, then its fine after that. Just did the cap, rotor, wires, and plugs, but it didn't fix it. Gonna do the coil tonight.

-Dave

[SpeedMetal]

Yep, the coil did it. It's running great. Now I just need to make it ride a little better....

[FullsizeFun]

I put a new coil in it and it is better, but not perfect. It still has a little stumble just off idle. But it doesn't do it nearly as much. It only did it once today. gonna put in a new fuel filter and check for any vacume leaks.

[tkr]

I put a new coil in it and it is better, but not perfect. It still has a little stumble just off idle. But it doesn't do it nearly as much. It only did it once today. gonna put in a new fuel filter and check for any vacume leaks.

Sounds more like a MAP sensor....

[FullsizeFun]

I just put in a MAP sensor and it didnt help. It is doing the same thing, idles fine but just above idle it will stumble or run rough. then it is fine above that. Any more ideas? distributor? Timing? Timing chain? It has 204K miles on it and the motor has never been touched. still sounds good, no valvetrain noise at all.
It is a 92 GMC 2500 5.7L
Thanks
Dave

[matt_helton]

dave, deffinetly check the timing and make sure the distributor is tight. but id look at the EGR and solenoid. sounds like it could be dumping when it should not be. ive gone through a few solenoids over the years on my big block for the same thing, it would idle fine and then you give it a bit of throttle to go it would stumble and fall on its face.

[FullsizeFun]

Matt
I dont think my truck has an EGR, and the distributor is tight and has never moved. Its getting pretty bad. It does it worse when it is cold. when its warm it only does it sometimes.
Thanks
Dave

[FullsizeFun]

and what is this called. it has 2 vacum lines and a 4 wire plug and a little filter on it that is dirty as hell. the number on it is 10068570.

[FlyHiFlyLo]

Sounds like a suck injector. Does it stumble when you are just sitting in the drive way and the truck is NOT under a load? Remove the air cleaner and climb up in the engine compartment and manually rev the engine. You should see an even spray comming from Both injectors in full throttle. If the spray is even hook an Ohm meter up to the throttle position switch located on the Throttle body. A chiltons type manual will spec out the proper ohm reading. there is two Torx screws that hold the TPS in position. loosen those and set the corect ohm level with the ignition in the off position and wide open throttle you will be able to tell if the TPS is bad. Buy a book and check these two things out prior to spending any more money.

[FullsizeFun]

It only does it when taking off from a stop and i give it just a little throttle. but if i mash the throttle it wont do it. It doesnt do it in the driveway. The injectors look good, they are spraying evenly. I'll check the TPS next. Thanks.
Dave

[matt_helton]

and what is this called. it has 2 vacum lines and a 4 wire plug and a little filter on it that is dirty as hell. the number on it is 10068570.


dave, that is the EGR control solenoid. that is what i was talking about me having to replace a few times.

[FullsizeFun]

thanks Matt
I'll get a new one today

[FullsizeFun]

Problem solved, it was the soleniod. Good lookin out Matt.

[matt_helton]

word up son.