4.3 to 5.7 TBI problems...

[BigJohnson]

I am helping a friend troubleshoot an engine miss in a 1991 pickup. The truck was originally a 4.3 TBI and they swapped in a brand new 350 TBI crate motor from GM. The truck is a manual trans 4x4. The ESC and knock sensor were swapped to 350 parts with brand new parts from the dealer. The ECM is a 1227747 with 350 prom. It also has a new 02 sensor and new distributor from GM. It even has a brand new throttle body complete with injectors from GM. The coil and coolant temp senosr are original.

The problem is the truck has had a what seems like a miss that causes the motor to shudder in 4 and 5th gear at low speed or under load. It idles normally most of the time with an ocassional miss at idle. You can feel the motor skip and load and unload the drivetrain. The engine otherwise runs reasonably well and has decent power.

The plugs and wires have been changed. We also tried a new ignition module, coil and rotor and it didn't help. We have scanned the truck with Winaldl and all the sensors seem to operate normally. MAP is normal, tps is smooth and seems reasonable. The BLM fuel numbers look normal and it is not going lean. The coolant temp we tested with for resistance and compared to values in the GM service and check out OK. The truck has NO trouble codes.

We then checked the timing. With the EST bypass disconnected the base timing was around 20-25 degress, well past the timing tab. We loosened the distributor and brought it down to zero degrees base time reconnected the bypass and cleared the codes. The truck ran even worse and still missed. It also threw code 43 for ESC module. We changed the module and didn't get another code. We then swapped back to the original one and didn't get another code. I believe the code 43 resulted from the ECM testing the module by advancing the timing to listen for knock and the motor didn't knock due to retarded timing although it was set to zero according to the timing tab. This makes me think the timing is off but I don't understand how it could run so well at times with the timing that far off.

We then bumped the timing back up to 12 degrees and the it ran about like it did before and still had the miss. We are going to re-time the motor tomorrow and verify TDC to try and figure out why the seems to need too much base timing. The only other thing we haven't checked is the fuel pressure and we are going to check that tomorrow as well.


Sorry for the long post but we have beat this thing to death to no avail. Any ideas on the timing or anything to try or look at would be greatly appreciated.

Thanks

 

[southernspeed]

I once had a hell of a job timing my truck because the outer iron ring on the damper had spun round 20-30' when I'd previously bent a rod! Checking your TDC to the timing mark will rule that out. And the fuel pressure you're going to check is just about all that is left to check by the sounds of it.
Post up when you find it, I'd be interested to hear what it is.
Good luck!

 

[BigJohnson]

Thanks for the reply!

I haven't had a chance to look at the truck today but my friend talked to a mechanic that gave up some insight on the problem. He thinks the problem is a lean miss on two cylinders caused my a failing fuel pump, dirty injectors, or a bad pressure regulator. The timing is not set for TDC to be at zero on the tab but I think it is close enough to not cause this problem.

Since the throttle body and injectors have been changed and the problem happened with both the new and old unit I am thinking it is a fuel pump or delivery problem.

Also, last night when we drove the truck it didn't miss when it was cold and got progressively worse when it got up to temperature.

We are going to check the fuel pressure and change the filter tonight. Also, I am going to swap my injector pod on the truck to rule out regulator or injector problems.

Will update when I find something out.

Thanks

 

[Thunder]

Sounds like it may be a magnetic pick coil up problem to me. Have you tested it? Pop the dist cap and unplug it from the module. Readings between the green and white wires should be 500-1500 OHMs. Reading fron either wire to ground should be infinite resistance. If out of specs replace.
The problems you describe sound exactly what they do when they start to go out. Thet will also throw a code 42 and light the CEL sometimes.

 

[sweetk30]

also what spark plugs are in it? if not ac/delco then change them. my 96 vortec 350 run o.k. and skiped under load with anything other than ac/delco.

 

[dirtwarrior17]

I am having this problem right now on my 355.... runs a lot better at around 16-20 degrees advanced than it does at say 4 advanced... I pulled the #1 plug and found the electrode on the spark plug had bent to touch the plug so basically the #1 cylinder wasn't firing... I pulled the electrode back out and gapped it and it ran better but wouldn't run at 4 degrees like it should. I found a giant vacuum leak at the back of the rtv seal between the manifold and the block because when i installed the manifold i sinched a part of the plastic on the harness in between them by a couple millimeters and then pulled it out and torqued it down again hoping the rtv would seal up but it didn't. I can even hear it whistle when i get on the throttle and it idles all over the place and sometimes will just die like i killed it.

Might want to have a look at the plugs incase one of them was dropped and got bent...real easy to do. also start it up and spray wd-40 or carb cleaner around areas with a possible leak to see if the idle changes.

my 2 cents...

shane

 

[86chevybanshee]

ill put money on the regulator or the pump.

my 92 s10 with a 2.8 did the same. i replace all the ignition parts. cleaned the injectors, did a compression check on all the cyinders, then i tested the fuel pressure and it was staying at a constant 15 psi (only suppose to be 9-13), so i put a new regulator in, still missfired, put a new pump in and bam! it was fixed

 

[4xcrazy]

i would say the pump as well, had the same type of problem with my 350 and it progressivly got worse, till i finally changed out the pump, not a problem since.

 

[BigJohnson]

Thanks for all the replies!

The miss is fixed!!! As it turns out the brand spankin new set of AC delco plug wires were the problem. When we finally got around to re-timing the motor, it shows 10 degrees BTDC at actual TDC btw, we found that the boots on the plug wires that go to the distributor had cracks in them that were only visible when you twisted them around. I have no idea why they were cracked but it was causing the problem and since they were brand new we were overlooking them. Just goes to show even if its new it doesn't mean its not defective

The truck runs great now and has much more power.

Never overlook the simple stuff

Thanks again for all the suggestions.