Manager Forum

I could use a litle help with some places to start on the cheap. This is a truck our shop just inherited. 211K miles. A/T 4x4 5.7 fuel inj. (two injectors at manifold).
This truck runs good at idle and such but is weak on power and especially after it get up to third gear or OD. as it hits overdrive especially, the power just drops off, like its dragging a loaded trailer or something. Every once in awhile you'lle feel the proper amount of power show up for maybe 2-4 seconds if your foot is on the throttle in just the right possition, but it will cut back on power again.
The plugs,cap rotor and even distributor was replaced a couple months ago by us to get the truck running for the prev. owner. but it was just way overdue for a tune up. But it ran like this before too. I have suspected the cat conver. but the problem never is changed by how long you drive and also, it will climb a hill. ( the exhaust can breathe) so ive ruled that out. Both injectors seem to always be working. Ive seen one die and run on just four cylinders before on other trucks and i do not think that is what is happening. I cannot test fuel pressure easily on this as I dont have the adapter to tie in easily but normally would ckeck that for 15 psi . The truck never dies and is pretty lame with a power brake. Where should i go from here?
It just seems like the whole relationship between timing, fuel, and gear ratio are all confused and thought I could ask for your guys opinions before i invest so much on this big dinosaur
Ps the trans does seem to work as it should.
Another reason fro this post is we just got in a 2000 4.3 jimmy with the same exact symptoms from another shop and its already been thru the ringer, and they gave up. So Ive got two GM's to fix now.
Thanks for any pointers of if Im overlooking the obvious. Chris

Unplug the knock sensors and see what happens. I think if you look under the dash on the passenger side you'll see a brown wire dropping out of the harness that runs along the rug line. Unplug it and throw a timing light on the truck and see where the timing sits and more importantly if the mark jumps around. If it is there is a possibility it needs a chain. Another issue these have is a bad egr amplifier that will stick and prematurely opens the valve causing the power to flatten. I would also attach a pressure gauge and watch the pressure to see it it's fuel related.

My approach for this would be......

Confirm mechanical engine. Check compression, timing, etc. Lots of miles means higher chances of something amiss.
Monitor 02 sensor under load. If lean, put a fuel gauge on this beast and confirm pressure and volume.
TEST backpressure in exhaust. Don't make any assumptions here. (don't ask me how I know, just say I've learned the hard way )
Then it's on to fuel and timing control. Injectors, coil etc.

OK. I will spend some time on this over the week and post results next week.

This is a very late (overdue) post . I did spend a little time rechecking the egr but no problems found. It took a while to find the timing wire, but after checking the timing, it was so far retarded, and that was the problem. Often its the basics. I think they had been dring this truck with it timed wrong for years. I should have checked this sooner instead of jumping to other conclusions....
The gmc Jimmy was not the distributor timing as it had the "fixed" type distributor. They took it back to where the heads were installed and I have not heard what was found. I'll try to post later.