91 5.7 TBI hesitation problem

[itsallhistory]

Hey Everyone,

I have a 91 V2500 with a 5.7 350 TBI motor (4x4 with 4L80E transmission & NP241C transfer case).

The problem I'm having is that when the Suburban warms up it starts hesitating when at cruising speed (just about any speed above 40 mph). It acts like its losing power & acceleration and I have to stomp on the gas to get it to go back up to speed.

I've changed the fuel pump, plug wires, throttle position sensor and it's still doing it. After changing the throttle position sensor it seemed to be fine for a day or two but now its doing it again.

Could it be the egr valve, fuel pressure regulator, O2 sensor? I don't want to throw parts at it. The service engine light has only come on once since I've owned it and that was about a year ago. It hasn't come on any of the times I've had this hesitation problem occur.

Any information/advice is greatly appreciated.

 

[89Suburban]

Pull the vacuum line off the EGR and plug it and see if it still does it.

 

[itsallhistory]

I pulled the hose off the egr. No change in idle speed.

 

[89Suburban]

I pulled the hose off the egr. No change in idle speed.

No I meant to see if drive-ability improves/off throttle acceleration. Did you warm it up and test drive?

 

[itsallhistory]

No. My son has it and is out driving right now. Should I pull the hose and plug the EGR or just leave the EGR line open and then test drive it?

 

[89Suburban]

No. My son has it and is out driving right now. Should I pull the hose and plug the EGR or just leave the EGR line open and then test drive it?

Pull the hose off the EGR and plug the hose. This is just a test. It may not make a difference but it is always on the top of my suspect list. If no improvement I would next try to pull codes off the computer. If it gets that far we will show you how.

 

[itsallhistory]

OK thanks. I'll plug the hose coming off the EGR and drive it tomorrow and see if that makes a difference. I'll post tomorrow and let you know what it does. Thanks.

 

[Jims86]

Have you changed the fuel filter?

 

[itsallhistory]

Yes. I changed the fuel filter along with the fuel pump. I bought a rebuild kit for the TBI in case it might be the fuel pressure regulator. I'm gonna try isolating the EGR valve today and see what happens. I'll post later with an update.

In the mean time, I appreciate the suggestions. All ideas are appreciated.

 

[itsallhistory]

Update: I think the problem is fixed. I disconnected the EGR valve from the EGR vacuum solenoid. There was no difference in the acceleration. The loss of power was still there. I then disconnected the vacuum solenoid from the TBI and there was no loss of power of drop-off in acceleration. So, I put a vacuum cap on the EGR valve and the port at the base of the TBI and it seems to run fine now.

 

[89Suburban]

Wow, what do we have going on here? How can a solenoid that is disconnected from the EGR valve cause this condition? I wouldn't think there is a big enough of a vacuum leak thru that valve to cause that problem. Huh. Well, if it does, then maybe the diaphragm in the EGR is busted. I'd like to hear Jim's thoughts on this one. Can you get a pic of the label on the radiator with the vacuum diagram and specs? So we can see what type of EGR you have? Then we can test it and try and trace down what is going on here.

 

[Jims86]

Wow, what do we have going on here? How can a solenoid that is disconnected from the EGR valve cause this condition? I wouldn't think there is a big enough of a vacuum leak thru that valve to cause that problem. Huh. Well, if it does, then maybe the diaphragm in the EGR is busted. I'd like to hear Jim's thoughts on this one. Can you get a pic of the label on the radiator with the vacuum diagram and specs? So we can see what type of EGR you have? Then we can test it and try and trace down what is going on here.

Sounds to me like another uncommanded EGR opening....bad solenoid. Wouldn't open if the diaphram was bad.
If the solenoid is bad, and he just unhooked EGR, a vac leak was created, when he plugged it at the T body, he took care of the leak.
Kevin, get a length of vac line to hook to the solenoid, at the end that the throttle body hooks to, and with the solenoid unplugged, see if you can suck through it(EGR side open)
If so, solenoid is bad.

 

[itsallhistory]

I will try to do that today sometime but I don't know if it will help. The diagram may not be the correct one for the engine. Here's why...

A little history on the suburban. It's titled as a 90 GMC R2500 but it's a 4x4. I had some transmission work done on it last year and the owner of the shop said that the tranny is a 4L80E so it has to be a 91. He looked on the driver's side door, and sure enough, there was a vin number from a 91 V2500 Suburban. So, his thoughts were that some time in it's past, a body from a 2wd Sub was placed on the 4wd chassis.

I do know that it was possibly some kind of fleet vehicle in Roswell NM. The AC system was retrofitted to R134A in 1999 and the label on the retrofit is from a shop in Roswell.

I will try to get a picture for you soon.

 

[Jims86]

I will try to do that today sometime but I don't know if it will help. The diagram may not be the correct one for the engine. Here's why...

A little history on the suburban. It's titled as a 90 GMC R2500 but it's a 4x4. I had some transmission work done on it last year and the owner of the shop said that the tranny is a 4L80E so it has to be a 91. He looked on the driver's side door, and sure enough, there was a vin number from a 91 V2500 Suburban. So, his thoughts were that some time in it's past, a body from a 2wd Sub was placed on the 4wd chassis.

I do know that it was possibly some kind of fleet vehicle in Roswell NM. The AC system was retrofitted to R134A in 1999 and the label on the retrofit is from a shop in Roswell.

I will try to get a picture for you soon.

Also, pull the glove box down and see if you can get us a part number on the computer.

 

[itsallhistory]

Thanks for the help everyone. I disconnected the vacuum solenoid and plugged the EGR port and the TBI port. The hesitation problem went away for about a week and came back yesterday (the ports are still plugged, btw). The only thing I can think to add is that it started raining yesterday (constant over several hours). Would the moisture have an effect in some way? Why would plugging the the EGR fix the problem temporarily? Could it be a crack in the disributor cap letting in moisture? Maybe the catalytic clogging up? Fuel pressure regulator in the TBI going bad? Coolant temp sensor? Like I said I don't want to just throw parts at it but I'm stumped.