1991 chevy 1500 5.7 tbi erratic idle

[NEUMANNZZ]

Originally Posted by FordeatinZ71

ok, maybe i've missed this...how are you running a '78 cam in a '91 block? the '91 is a roller cam...the '78 is not...again, maybe i missed it as i just skimmed this.

are you sure it's firing on all 8? might wanna do a compression test. have you replaced the coil or ignition module yet? those can both cause weak spark and the ignition module can cause the injectors not to work right.

91 is not a roller cam.

[NEUMANNZZ]

Originally Posted by tpulliam79

well put a stethoscope on the dizzy and it sure doesnt sound right seems like it makes a weird sound right before the idle gets funny.. how do you use a timing light?

its easy hook it up to the battery(red wire to red +/ black wire to black - battery terminals). then hook the induction pickup to # 1 spark plug wire(be careful not to burn it on exhaust). start engine point timing light to balancer and timing tab. pull trigger and enjoy the light show(light will flash). balancer mark will line up with timing tab zero degrees. if not rotate distributor slowly until it lines up(loosen holdown bolt until its just snug)

[tpulliam79]

the block is from a 78 the heads are from the 91 cam is from the 78

thanks for the heads up on the timing procedure. once I get a light i will check it out

[dunnroadmafia]

i am having this exact problem with my 93 tbi 350 and my mechanic, dad, uncle and damn near every one i know thinks its the distributor. So please keep us posted and good luck

[tpulliam79]

well my mother has fallen on some bad times so all my extra money is going to her for now, but as soon as I get through this and my six year anniversary I will be working on it again. I will post as soon as I know something or run into more questions

[FordeatinZ71]

Originally Posted by NEUMANNZZ

91 is not a roller cam.

well i'll be damned! i always thought 87-up motors were roller cammed...learn something new every day, thanks man!

[tpulliam79]

dumb question... the power steering doesnt affect vacuum does it? seems like the idle gets way off anytime im stopped and turn the wheels?

[tpulliam79]

bump

[frank1]

check your egr valve for carbon build up. the egr valve should be closed at idle. the carbon will not let it seat all the way closed and it won't set a code. Just had the same problem with my 2000 k2500 5.7. good luck. Frank

[NEUMANNZZ]

Originally Posted by tpulliam79

dumb question... the power steering doesnt affect vacuum does it? seems like the idle gets way off anytime im stopped and turn the wheels?

No, but it will put a load on the engine.

[tpulliam79]

so i have my anniversary coming up and my wife is going to buy me whatever I want and it looks like its going to be parts..... when we did the intake gasket the egr passages were completely clogged with carbon, obviously we cleaned them out think its safe to assume that there is a lot of carbon build up in the egr iself? also would the egr cause low vaccuum at idle? and if I recall correctly when my mechanic was checking timing I dont think it changed after the est wire was reconnected, what would cause that? the knock sensor wasnt hooked up for some time, is there a way to check this? here are all the new parts already in.... themostat, o2, plugs , wires, cap, rotor, icm , coil, cts,intake gaskets, tbi all new gaskets and cleaned, tps , map, and iac ,

heres what I was thinking of getting ( still fighting a rough idle and now it seems to miss when cold)
distributor, knock sensor, egr valve, egr solenoid, oil pressure sensor, and a timing light

think im chasing my tail here?

[NEUMANNZZ]

your egr could affect vacuum if the diaphram is ruptured/sticking or the EGR selenoid is stuck open.

timing should advance with the est wire plugged in and code 42 reset. at idle it should be around 20 degrees. when you give it gas it should advance further.

the ECM controls spark advance with the est system. to properly control ignition the ecm needs to know
cranshaft position
engine speed
engine load(manifold vacuum)
atmospheric pressure
engine coolant temperature.

tbi distributor 101:
the four post connector to the distributor:
pin A: reference ground low, may be ground to distributor, it makes sure the ground circuit between ecm and module. if open may cause poor performance.

pin B: bypass. at about 400 rpms the ecm applies 5 volts dc to this circuit to switch spark timing control from the module to ecm. an open or grounded bypass circuit will set code 42 and the engine will run at base timing, plus a small amount of advance built into the module.

pin C: distributor reference high. this provides the ecm with rpm and crankshaft position info.

pin D: EST. this circuit triggers the module. The ecm does not know what the actual timing is but it does know when it gets the reference signal. it then advances or retards the spark from that point. therefore if base timing is set incorrectly the engine spark curve will be incorrect.

results of incorrect est operation:

low map output voltage: more spark advance
cold engine: more spark advance
hot engine :less spark advance
high map output voltage: less spark advance

detonation can be caused by high map output or high resistance in the coolant temp sensor circuit.

poor performance can be caused by high map output or low resistance in the coolant temp sensor circuit.

DIAGRAMS HERE:

http://www.fullsizechevy.com/forums/...3&d=1206741675

http://www.fullsizechevy.com/forums/...4&d=1220576357

PICKUP COIL AND COIL TEST:

http://www.autozone.com/az/cds/en_us...rInfoPages.htm

[tpulliam79]

ok just got back from Meineke for some warranty work, they are telling me the cat is bad and this is why im having issues. another shop tells me that the cat and muffler cant affect anything at idle? im confused. also whoever had truck before me welded in a 2 inch muffler and all the other pipes are 3 inch obviously causing restriction. the muffler is going to be replaced in the next couple days, had the cat backflow tested and they said its 1 lob and is not plugged. could the cat being bad ( not working right) and the wrong size muffler affect anything at idle?

[NEUMANNZZ]

If the cat is plugged up(excessive backpressure) yes you can have problems, but usually not at idle, usually under load. a 2 inch in/out muffler shouldnt cause an issue unless its plugged up too.

[NEUMANNZZ]

just becuase they said the cat is bad probably means you wont pass emmissions.

[tpulliam79]

so the cat and muffler are not affecting the idle then you think

[NEUMANNZZ]

probably not.

[tpulliam79]

well its got to be fixed either way so im gonna get the exhaust all redone with single in and dual out new pipes and tips.. the shop owner says hes 99% sure that this wiull fix my problem I told him if it doesnt then he has to diagnose it for free...

[C/K1500]

1. There is nothing wrong with your cat. The motor has no idea if there even is one, since it is mounted behind the O2 sensor. If it is plugged, you will lose pwer on the highway. At idle there would be no issue.

2. You have not mentioned if your injectors are spraying nicely or dribbling fuel. If they are dribbling, that could be your problem.

3. You could have an air leak at the throttle shaft. Grab it and see if there is play. If there is, see if the play is the same at different throttle openings. Most wear occurs (and the most impact to drivability) at the throttle tip-in point.

4. Make sure you ignition wires, etc. are good. SOmeone pasted an Autozone link and that shows you good ways to rule out an ignition problem.

5. Your vacuum is too low at idle. 1978 Chevys idled at 21 inches of mercury. There is no problem with your cam (unless of course it has flat lobes). Since you have no problems at higher rpm or under load, I would rule out the cam as a problem. You might still have a vacuum leak due to improperly installed manifold gaskets or a warped manifold. Since you feel that the mechanic who reinstalled the manifold gaskets was a gibroni, perhaps he did not do this job well.

6. A bad EGR valve will cause drivability problems, such as excessive pinging at speed. I have seen brand new, faulty EGR valves right out of the box. These engines need exhaust recirculation to run well.

7. A bad knock sensor will cause - your engine to knock (ping) like crazy. You have not reported that as a problem, so look elsewhere.

8. You should have a 195 degree thermostat. That is what your ECU is calibrated for. The warmer the better for driveability and economy. For drag strip performance, yes, go cooler. But not on a street car or with today's gas prices.

9. To set the timing with a timing light, you must disconnect that wire as described in a previous posting. Set the timing as far ahead as you like. It doesn't matter that much, as long as the engine does not ping once you reconnect everything and drive it. You will notice that retarding the timing tends to give a smoother idle, but the engine will have no balls. Advancing slightly, the engine idles with a bit of a 'roll' or occasional hiccup, but has more power. In general, run the most timing for the grade of gasoline that you use that does not cause the engine to ping under the type of driving you normally do.

10. A bad MAP sensor could cause problems. Did you already change yours? I believe that you did. But a MAP sensor is not the first place I would be looking as they tend to fazool things farther up the power band.

Lastly, remember the first law of auto repair shops: the problem you have can only be fixed by whatever high-margin parts that shop sells. So, if you take the truck to Meineke, it will need a new cat for whatever problem you showed up with. As so on and so forth.

Good luck, and let us know.

[C/K1500]

1. There is nothing wrong with your cat. The motor has no idea if there even is one, since it is mounted behind the O2 sensor. If it is plugged, you will lose pwer on the highway. At idle there would be no issue.

2. You have not mentioned if your injectors are spraying nicely or dribbling fuel. If they are dribbling, that could be your problem.

3. You could have an air leak at the throttle shaft. Grab it and see if there is play. If there is, see if the play is the same at different throttle openings. Most wear occurs (and the most impact to drivability) at the throttle tip-in point.

4. Make sure you ignition wires, etc. are good. SOmeone pasted an Autozone link and that shows you good ways to rule out an ignition problem.

5. Your vacuum is too low at idle. 1978 Chevys idled at 21 inches of mercury. There is no problem with your cam (unless of course it has flat lobes). Since you have no problems at higher rpm or under load, I would rule out the cam as a problem. You might still have a vacuum leak due to improperly installed manifold gaskets or a warped manifold. Since you feel that the mechanic who reinstalled the manifold gaskets was a gibroni, perhaps he did not do this job well.

6. A bad EGR valve will cause drivability problems, such as excessive pinging at speed. I have seen brand new, faulty EGR valves right out of the box. These engines need exhaust recirculation to run well.

7. A bad knock sensor will cause - your engine to knock (ping) like crazy. You have not reported that as a problem, so look elsewhere.

8. You should have a 195 degree thermostat. That is what your ECU is calibrated for. The warmer the better for driveability and economy. For drag strip performance, yes, go cooler. But not on a street car or with today's gas prices.

9. To set the timing with a timing light, you must disconnect that wire as described in a previous posting. Set the timing as far ahead as you like. It doesn't matter that much, as long as the engine does not ping once you reconnect everything and drive it. You will notice that retarding the timing tends to give a smoother idle, but the engine will have no balls. Advancing slightly, the engine idles with a bit of a 'roll' or occasional hiccup, but has more power. In general, run the most timing for the grade of gasoline that you use that does not cause the engine to ping under the type of driving you normally do.

10. A bad MAP sensor could cause problems. Did you already change yours? I believe that you did. But a MAP sensor is not the first place I would be looking as they tend to fazool things farther up the power band.

Lastly, remember the first law of auto repair shops: the problem you have can only be fixed by whatever high-margin parts that shop sells. So, if you take the truck to Meineke, it will need a new cat for whatever problem you showed up with. As so on and so forth.

Good luck, and let us know.