Idles very rough, unless I undo the MAP sensor

DREWSKIE3687

MEMBER
1987 CHEVROLET SUBURBAN
5.7L
V8
4WD
AUTOMATIC
600 MILES

The truck always idled rough, even after I put a new long block engine in. I can't keep it idling, unless I unhook the vacuum line from the map sensor. When I do that, it cranks no problem and idles smooth. But I can barely drive it around the block that way, as it wants to bog down under a load. When it starts to bog the most I notice it puffs white smoke out of the exhaust. Also, the oil pressure stays high, between 50-60 on the gauge.
Since I've had the new motor, I put about 600 miles on it and then I overheated it and warped the heads. That's when the problem got worse. I replaced the water pump and heads and many sensors, O2, TPS, knock, oil pressure, temperature, EGR and MAP sensor.
When I remove the hose from the map sensor, the truck stores code 33. If I unplug the brown wire that puts it in "base timing mode", it also stores code 42 but doesn't seem to affect the idle at all. This is how I had it when I set the timing to 0°.
Any help is appreciated, I've been tinkering on this for over a year now to no avail.

8 Replies

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

Lets give this a try. Hopefully between the two of us, we can figure this out.

Are any codes present when everything is plugged in as it should be? The idea that you get a 33 when you unplug the MAP makes sense simply because it is a low vacuum/high signal voltage.

The 42 as I feel you already know is related to the electronic spark timing. So that makes sense as well.

Other thank the rough idle, does it run properly when the map is connected? I have test procedures for the codes you have, but I suspect they aren't useful simply because you are creating the codes by disconnecting things.

I assume things such as vacuum leaks, the idle air control valve and such components have been tested, correct?

Let me know.
Joe

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Sunday, August 30th, 2020 AT 7:38 PM

DREWSKIE3687

MEMBER
5 POSTS

Thanks for the reply.
It doesn't throw any codes when I have everything hooked up. But I have to keep feathering the gas pedal to keep it idling and sometimes it'll puff air/fuel out of the throttle body and shut off instantly, like the timing is way off. I've checked for vacuum leaks. The IAC is, I think, the only sensor or valve I haven't replaced, but it looks pretty new. The guy who had the truck before me was going through the same stuff apparently, as I've replaced a few "new" sensors. But I figured that would probably throw a code if the IAC was bad? So, at this point, I'm leaning towards the fuel pump? But it idles so good without the MAP sensor, makes me think it's in the ignition or vacuum. But mostly all that stuff is new and no leaks that I can find, I used enough carburetor cleaner to almost catch it on fire. So, I don't know. I don't have a fuel pressure gauge, so I think I'm going to change the fuel pump this weekend and cross my fingers.

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Monday, August 31st, 2020 AT 8:46 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

If the IAC is bad or carboned up, it may not throw a code.

Here is a link you may find of interest:

https://www.2carpros.com/articles/idle-air-control-valve-service

1987 Chevy Truck Suburban 1/2 Ton 4WD V8-350 5.7L
Rough, Unstable, or Incorrect Idle, Stalling
Vehicle Power-train Management Computers and Control Systems Testing and Inspection Symptom Related Diagnostic Procedures - Diagnosis By Symptom Rough, Unstable, or Incorrect Idle, Stalling
Rough, unstable, or incorrect idle, stalling.
1. Perform thorough visual inspection as outlined in "Intermittent Condition Diagnosis".
2. Check Ignition timing and ensure ignition system components are satisfactory.
3. Check for binding or sticking TPS and throttle linkage.
4. Check IAC valve and control system.
5. Ensure charging system output is within specifications.
6. Check park/neutral switch circuit.
7. Check operation of EEC system and canister purge control.
8. Check power steering pressure switch circuit.
9. Check EGR system for condition causing valve to remain open or partly open at all times.
10. Inspect oxygen sensor for contamination. Contaminated sensor will cause ECM to reduce amount of fuel delivery.
11. Check Ignition timing and ensure ignition system components are satisfactory.
12. Check for fuel in pressure regulator signal hose. If present, replace regulator.
13. If problem exists with A/C on, check A/C request, A/C signal and IAC circuits with SCAN tool.

_________________________

Let me know if this helps.

Joe

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Tuesday, September 1st, 2020 AT 6:36 PM

DREWSKIE3687

MEMBER
5 POSTS

Thanks again for all the help, I'm making progress!
I changed the fuel pump and it made a world of difference, but still stalled when idling sometimes. So I took out the IAC and cleaned that all up, wasn't too dirty but it did make a difference in the idle, but still stalled randomly. So I turned the idle screw up a little bit and now it doesn't stall! But it still doesn't idle perfectly, it's like the computer keeps searching for the sweet spot but can't find it and causes it to miss randomly. I don't know.
Anyhow, I took her for a test drive. She's idled a little high, of course, but there's a lag when I give her throttle and she's lacking power. I got her on the highway and the check engine light came on. Codes 13 and 44, both dealing with the o2 sensor, but the sensor is brand new. I'm gonna put the voltmeter on it and see what I can figure out. Any other suggestions?

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Saturday, September 5th, 2020 AT 2:46 PM

JACOBANDNICKOLAS

MECHANIC
109,885 POSTS

Hi,

The code 13 is related to an open circuit in the O2 system. Confirm the connections is in good condition, the internal pins are not bent, corroded, or broken.

As far as the 44, that indicates a lean fuel mixture condition. Have you checked for engine vacuum leaks? Are there any exhaust leaks? An engine vacuum leak can cause an idle issue as well as a rough idle.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know.
Joe