1991 chevy 1500 5.7 tbi erratic idle

[20chevy00]

i saw a post on here about someones engine stumbling when idling. They changed the distributor gear and that fixed the problem. My tahoe does the same stumbling type stuff. I ordered a new gear this week and will be replacing it to see if that is the culprit. good luck.

[tpulliam79]

well the balancer and new distributor went in and it runs stronger and we thought we had the jumpy idle fixed let it run for 20 min no issues , got in the truck gave it some gas for a minute and then it went right back to the searching for idle. electreical current is good now im thinking its got to be fuel delivery of some type, any one have an idea why it would run so good until its revved then back to the crappy idle?

[loafycleetus]

Originally Posted by tpulliam79

dumps fuel at idle because the vacuum pressure is off. he never did check the fuel pressure.

what is the fuel pressure then? has the kinked line been fixed?

[tpulliam79]

the line has not been fixed but i don't think its the issue. all the work and testing the first mechanic did is moot as he said the distributor and egr were fine , I found the egr was shot and after pulling the distributor i don't know how the truck was even running, so all he told me is not safe to think was right so I am going to ohm out my injectors, and try to check them for leaks, then try to find a fuel pressure tester that will work with my tbi, It just baffles me because it ran so great until i revved it then it went right back to the surge? the fuel return line is the one that's kinked but its not that bad and fuel can still go through it. if all checks out im thinking maybe the hose on the fuel pump that's known for going south?

[mtrmike55]

Checking the fuel pressure WITHOUT fixing the return line kink will do you no good. The return line problem must be repaired. Have you checked the "idle air rate"? This is the adjustment for the throttle plate opening at idle. The adjustment screw is hidden by a plug and usually is not adjusted. If the plug has been removed than it has probably been adjusted, sometimes wrongly , to try to correct an idle problem.

[tpulliam79]

Originally Posted by mtrmike55

Checking the fuel pressure WITHOUT fixing the return line kink will do you no good. The return line problem must be repaired.

its not kinked shut just slightly twisted, i dont see how it would make a difference? i will see if i can get my buddy to go to the yard with me so I can have him crawl under and get one for me, i cant crawl under stuff just yet, hell I had my wife under the truck with me to do the starter ( one arm doesnt get it done.)

[tpulliam79]

anybody know, do I have to drop the tank to replace the fuel return line from the tbi?

[NEUMANNZZ]

starting from the beginning:

sensors:

iac valve: for smooth cycling passage clear of carbon, no wiring or connector issues, iac valve should cycle with diagnostic terminal jumped pin A to B, ignition on engine stopped. using scan tool, IAC counts at idle in correct range 5-30 in closed loop. if out of range check for vacuum leaks.
tps sensor: smooth linear response, no dead spots or voltage spikes. checked with scan tool or multimeter, around .5 to around 5 vdc at WOT.
coolant temp sensor: well above 1000 ohms when cold. around 100 ohms when hot. ensure proper thermostat is used. cooling sytem operating properly?
map sensor(with no vacuum leaks or wiring/connector issues) disconnect electrically. run vehicle? results?
o2 sensor: (wiring repaired with solder) check with scan tool cold then when in closed loop.

electrical:

wiring: check to ensure computer is grounded to engine(less then one ohm) and no variation to readings.
Check battery cables.
check for intermittent power/ fuzible links.
last resort check continuity for every single wire connected to ECM.

ensure alternator voltage is steady. if not test alternator and wiring/connections.

ignition:

check timing. ensure timing advances when engine revved.
check spark plug wires cap/rotor. also check wires and connectors for continuity.
est wire disconnected? test drive? results? if better
check coil and pickup coil with ohm meter use test in the manual. tests pass see map sensor.

fuel:

check for leaking injectors. ignition on,engine stopped. should be no leaks.
spray pattern when running should be a fine mist with cone shape barely seen. droplets okay. both injectors equal. if not check wiring continuity to injectors. check ignition control module(test multiple times.)
check fuel pressure when hot and when cold at idle. should be no variation greater then 2 psi. if so check fuel pump ground/wiring/connectors/regulator diaphram and spring.


emmissions:

check for fuel in vapor canistor.
check for overpressurization in gas tank.
egr valve isolation, remove and block passage. test drive.
check pcv valve for proper operation.
check for exhaust leaks.
Smog pump for proper operation. no leaks.


mechanical:
unusual drag on belt
check pulleys
compression test.
broken motor mounts.

other:

A/C compressor stuck on or if on high pressure switch faulty.
check park/neutral switch

[NEUMANNZZ]

Originally Posted by tpulliam79

anybody know, do I have to drop the tank to replace the fuel return line from the tbi?

i wouldnt mess with the fuel return line unless your pressure is unstable.

[tpulliam79]

what should the continuity be of the injectors and the wiring? also i moved the park neutral switch all the way one way and it seems to have smoothed out a little? Could very well be coincidence .

[tpulliam79]

any one know the continuity of the injectors and wires? both checked in at 2 and the wires had one: 1325 and the other 1190? are these numbers right? also where is the ground for the fuel pump located thanks. also the park neutral switch has two connectors and i believe from the pics online both spots on the switch should have male connectors for the connector to slide on to and there are none for one of the connectors?

[mtrmike55]

The fuel pump ground is a black wire that comes from the sending/pump assembly and is bolted to the top of the left frame rail near the tank. 13mm bolt ,I think.

[tpulliam79]

any body know what the continuity should be on the injectors? and on the wires going to the injectors?

[tpulliam79]

well I replaced neutral switch as it was bad and it helped a little , fixed the reverse light issue by adjusting the bar off the column, and it didn't change, It runs great until gas is applied then after you let off it starts the chugging ( not nearly as bad now) at this point i'm thinking the return line may have something to do with it. the way it is twisted I am wondering if once too much gas tries to go through it if its going back into the tbi and messing with it. I am going to take it off and see if this hose place in town can repair it, if not off to the yard i go and last effort ill pay 70 for a new one from the napa store. I will post back with results.

[Cheyenne'88]

I've read on here that some guys pinch that return line a little to raise fuel pressure. (poor man's AFPR)
Haven't read the whole thread, have you checked the fuel pressure?

[NEUMANNZZ]

the injectors are switched to ground by the ecm to fire. wires from injectors to ecm continuity should be under 1 ohm.

dont know what the injectors are supposed to ohm out at.

red and white wires are 12vdc. come from the inj a-b fuse. dark green and dark blue go to the ecm where they are switched/pulsed to ground via injector drivers.

[tpulliam79]

well after getting the return line repaired i lost my keys 4 hours later I found them . got the return line back on and so far so good , at idle it idles good just a little searching ( way better and I cant get it to do it ), it doesn't try to die anymore. when put in to gear it searches for a minute but then steadies out, wondering if from being kinked for so long the computer may think it needs less fuel? I am not convinced its solved yet but ill know in a few days....., I hope its fixed but we will see. before it was more in park now its just a little when first put in gear.

[NEUMANNZZ]

[BigBody90]

Do the TBI computers learn like that? If so, could you just unhook your battery cables for 10 munutes to reset it?

[tpulliam79]

Originally Posted by BigBody90

Do the TBI computers learn like that? If so, could you just unhook your battery cables for 10 munutes to reset it?

probably could just unhook the battery but after yesterdays fiasco with getting the truck unlocked and not having keys ( alarm was going off for about 10 min and more off and on) I want to give the neighbors a break