91 burb running rough need help ASAP

[80 Chevy K20 4x4]

My good friend has a 91 burb that has just had the TBI 5.7 engine gone through. It was running really rough because it needed new tune-up parts. I replaced the following parts;

1. Brand new ignition module
2. New plugs
3. New plug wires
4. New fuel filter
5. New Dist cap and rotor
6. Static timed the engine and then set the ignition timing to 0*
7. Plugged Cat removed and straight pipe installed
8. Compression checked at 145psi all 8 cylinders
9. Scan tool showed on code 42 in memory and the O2 sensor seemed to hover between 434mv-473mv in open loop. I never did see it go into closed loop mode either as the engine warmed up....
10. Cleaned the IAC solenoid
11. New main and rod bearings
12. New timing chain set
13. New oil pump

His burb has a slight miss at idle as I can feel it in the rig, and when you blip the throttle while it's in Park or Neutral, it will rev up normally. But if I have him power brake it or test drive it going up a hill, it will literally buck and hesitate until you let off of the gas pedal.

The fuel coming out of the injectors has a nice spray pattern to it. Between the 2 of us we don't have a fuel pressure gauge to see what the pump is running at, but he said he didn't have this problem until the motor was gone through and installed again into the burb.

I'm either leaning towards the fuel pump, the pick up coil in the distributor, or maybe a faulty O2 sensor.....? Anyone have any ideas for me to try next to resolve the issue?

Thanks for your help,
Doug

 

[chevyuser74]

id look at the egr system, ive seen a faulty valve do this more than once
it probably wouldnt be the oxygen sensor it could be the fuel pump but a simple gauge could tell u if you are having problems there, i doubt it is the ignition module, but like i said test the egr first

 

[4X4HIGH]

When you set the timing did you remember to disconnect the EST wire located near the brake booster?

It is possible that whoever built the engine did not use a TBI friendly cam. With a fresh rebuild and having only 145 psi cranking compression seems really low to me since my original stock 89 engine with 210K had 205 psi cranking compression. You might want to find out what cam was installed in this engine.

 

[chevyuser74]

it never says the engine was rebuilt

 

[4X4HIGH]

it never says the engine was rebuilt

I take this

My good friend has a 91 burb that has just had the TBI 5.7 engine gone through

as meaning it was rebuilt.

 

[chevyuser74]

ya i guess i could see that.

 

[mtn. burb]

When you set the timing did you remember to disconnect the EST wire located near the brake booster?

This is what I was thinking, many guys overlook this point.

Smog shop idiots forget this too, tell me my timing is waaaaay out. Even though the motor is purring. Unplug the EST please. "Oh, I guess it's good." No kidding!

 

[80 Chevy K20 4x4]

Okay I guess I should have been a little more specific regarding the engine "gone through" aspect.....lol

When my friend bought the burb it was running pretty good for about 3 days, then it lost a rod bearing. So the guy he bought it from is a reputable chevy mechanic and he told my friend to pull the motor and he'd replace the main and rod bearings, throw new rings in it, and a new timing chain. The mechanic left the factory stock cam in the engine.

Initially when he called me and asked me if I could look at it for him, I disconnected the EST connector to set the ignition timing after I checked the static timing to make sure that the distributor wasn't off by 1 tooth or something like that.

The EGR valve seems to be working correctly and is not stuck open.

 

[dyeager535]

You'd need to watch the O2 once the engine heats up, and it MAY require that the engine is revved up, or under load, before it starts working, since it requires quite a bit of exhaust heat to work correctly.

This is one reason heated O2's are suggested when using headers, although not required, *IF* you can verify that the engine goes closed loop with a scanner. Headers can bleed off enough heat that it the O2 won't work right. I tested this on mine, the stock replacement O2 does work correctly with my headers, but that isn't a given on every vehicle.

I have no idea why GM designed the system this way, but it was common at least into the '90's (from about 1981 when all cars got O2 sensors) for the system to drop out of closed loop at idle. Mine certainly does, but with ANY throttle input it goes closed loop immediately.

To summarize O2 sensor operation: if it goes closed loop and the O2 bounces over/under .500, it's pretty likely it's working correctly.

 

[80 Chevy K20 4x4]

I had my brother's scanner on the burb and I noticed the O2 Sensor hovering between 473mv - 438mv in open loop. I didn't see it go into closed loop mode when I had it idling and running to test things.

Is that the ESC module that sits under the air cleaner intake and sits in front of the map sensor? How can a person test it to verify if it's good or not?

Is there anything else in the distributor besides the pickup coil inner shaft piece and the ignition module that could cause stumbling under a load?

The burb has factory exhaust manifolds and exhaust on it outside of the cat has been eliminated.

 

[dyeager535]

Not sure where the ESC is on the TBI vehicles, someone will clue us in. I suspect it's a flat, square, black module with something like 7 wires going to it, on a flat plug, like my TPI ESC.

The ESC code truobleshooting links here http://coloradok5.com/forums/showthread.php?t=141717&highlight=troubleshooting will allow you to diagnose, but kind of grasping at straws if not throwing a code for EST (or ESC, same thing). There are troubleshooting charts for poor performance, I think my '91 truck manual shows that.

I suppose fuel pressure could be weak, but wouldn't that be indicated even at idle when the throttle was stabbed?

 

[4X4HIGH]

The ESC module is located on the passenger side of the throttle body with a connector just like Dorian said.

 

[80 Chevy K20 4x4]

I need an opinion on this;

Would severe stumbling, bucking, hesitation be more fuel related or ignition related?

The only thing we haven't been able to check yet is the fuel pressure or the pickup coil/inner shaft of the distributor....I just kinda want some others' opinions on this before I start chasing some more things on this burb.

Thanks,
Doug

 

[4X4HIGH]

I need an opinion on this;

Would severe stumbling, bucking, hesitation be more fuel related or ignition related?

The only thing we haven't been able to check yet is the fuel pressure or the pickup coil/inner shaft of the distributor....I just kinda want some others' opinions on this before I start chasing some more things on this burb.

Thanks,
Doug

Definately fuel related.

 

[chevyuser74]

Definately fuel related.

not necessarily it could be ignition system as well, for example if a sensor is wrong or lying to the computer...

 

[4X4HIGH]

not necessarily it could be ignition system as well, for example if a sensor is wrong or lying to the computer...

Did you happen to see what parts he's already replaced? Have you ever taken a good close look at all the post i reply to and see that i might just know a little something about what i'm talking about.

 

[chevyuser74]

Did you happen to see what parts he's already replaced? Have you ever taken a good close look at all the post i reply to and see that i might just know a little something about what i'm talking about.

just because u wrench on your vehicles in your backyard doesnt mean u know everything, my boss has owned his own shop for 34 years and even he wouldnt been so smug. even new parts fail and he really hasnt replaced alot of stuff.

 

[chevyuser74]

I need an opinion on this;

Would severe stumbling, bucking, hesitation be more fuel related or ignition related?

The only thing we haven't been able to check yet is the fuel pressure or the pickup coil/inner shaft of the distributor....I just kinda want some others' opinions on this before I start chasing some more things on this burb.

Thanks,
Doug

we had a truck in just last year for these symtoms it turned out to be a worn out throttle body, maybe not your problem but sometimes its something out of the norm

 

[4X4HIGH]

just because u wrench on your vehicles in your backyard doesnt mean u know everything, my boss has owned his own shop for 34 years and even he wouldnt been so smug. even new parts fail and he really hasnt replaced alot of stuff.

Son, you have alot to learn about me. Do some searching and you'll find out alot. Hell, i've probably been working on cars longer than you've been alive. I've also been a Professional Automotive machinist for 22 years.

 

[cuervo]

Son, you have alot to learn about me. Do some searching and you'll find out alot. Hell, i've probably been working on cars longer than you've been alive. I've also been a Professional Automotive machinist for 22 years.

And your smugness comes through loud and clear anytime someone does not agree with you. Rather then say you think otherwise you pretty much go into attack mode. I'm sure I live to regret this post, but meh, it's the truth.