Surging Idle: Thinking out loud

[wczimmerman]

Ok...pics are uploaded! Nothing of note, just a 2500 Suburban that serves me well.

Significant find today: since it wasn't raining when I got home, I thought I would do some more troubleshooting. So, I first disconnected the ESC module and started the truck-same surging in Park. After reconnecting it, I then unplugged the MAP sensor and restarted-NO SURGING! The idle was a bit lower, too. Now, that MAP sensor has been replaced recently and it was surging before and after the replacement so what does this mean? Aftermarket poor quality? I can get an AC Delco unit and put it on the truck easily enough. There seems to be a question around the GVW >8500lbs (I'm pretty sure mine is over that in rating) in terms of which MAP sensor would be the correct one.

Any thoughts from anyone on this?

 

[1987 GMC Jimmy]

The unplugging test is a good start, but I'm going to attach an article with more tests so you can be sure. Any item from the parts store is not exempt from the potential for failure. If anything, many of them are more failure prone than the OEM counterpart. Is the sticker on the driver door still in place? That should give you GVWR, what tires came with it, production month and date, etc.

http://easyautodiagnostics.com/gm/2.8L/map-sensor-multimeter-test-1

 

[wczimmerman]

So, if the MAP sensor passes the tests what would/could be the next thing to check? I haven't had the time to do the tests yet but I want to be thinking ahead...

The vacuum hose from the MAP sensor to the throttle body has been replaced and it is in good condition, incidentally.

 

[1987 GMC Jimmy]

From mine and others' experiences with TBI, I'd check that the ground strap is in good shape between passenger side cylinder head and the firewall. The Coolant Temperature Sensor can cause issues like this; the truck always runs rich when that goes bad. If it looks old, it may be a good idea to just replace it because they do wear out over time. You could test the EGR valve. I've had a range of experiences with that one. From it running like garbage driving down the highway and dying to it just causing a vacuum leak and poorer performance. The old style EGR valves had an open plunger, and the new ones have a sealed plunger with little holes. If you could find a way, depending on whether you have the old valve or a newer one, to reach under it and depress the plunger, cap your finger over the nipple, and let go of the plunger, that's the EGR test I do. If the plunger stays in place, the valve is good. If it falls back down, it's bad. The tuneup is a good idea, too. Worn plugs can cause some issues, but if a cap and rotor have been let go of long enough, they also can. I think those should fix whatever's going on if the MAP sensor stuff doesn't work out, but it looks like you've isolated the issue.

 

[Rusty Nail]

Pulled the codes lately? You have a paperclip, right?

 

[wczimmerman]

Did a little test this evening by first trying to pinch off the vacuum line to the MAP sensor and see if that made any difference. Surprisingly, it didn't. However, if I pull the vacuum line completely while the truck is running it dies instantly. For my final test, I pulled the electrical plug on the MAP sensor and then drove the truck for an errand (the CEL was on, of course). The truck frankly ran fantastic. Even the transmission was shifting smoother. Doesn't the computer go into some sort of "limp home mode" when the MAP is disconnected? I will likely order an ACDelco MAP sensor from Amazon at this point but I thought I would share these findings.

 

[1987 GMC Jimmy]

Yeah, it'll go into limp home mode and defer MAP input to the TPS sensor. It basically richens up the AFR and simplifies functions so the truck can run and make it to a stop without breaking down when it detects a problem. I feel like it's more than likely the MAP sensor malfunctioning, but I'm not a hundred percent. It could be the wiring. If the voltages at the connector are within spec with the key on, engine off and key on, engine on, I think it'd be a pretty safe investment. I just don't want you to pay that pretty penny for an AC Delco sensor, and it not fix the problem.

 

[wczimmerman]

I drove the truck for a few days with the MAP sensor disconnected and I had no surging idle. Saturday I installed a new ACDelco MAP sensor: idle still surging. This evening I installed a new ACDelco coolant temp sensor: idle still surging.

To recap: the idle only surges when in Park. In Drive, it doesn't do it. If I coast at idle speeds it will eventually surge slightly when you reach a certain speed (I'm guessing about 3 mph?). I'm not sure where to go from here. If it was a vacuum leak I would think that it would do it even when the MAP was unplugged. But since the MAP measures vacuum, maybe that could be it anyway?

 

[1987 GMC Jimmy]

I've seen similar issues like this one with a bad ECM. No trouble codes, right? Does the SES light come on when you turn the key on?

 

[wczimmerman]

I've seen similar issues like this one with a bad ECM. No trouble codes, right? Does the SES light come on when you turn the key on?

When I start the truck? Yes.

 

[wczimmerman]

I've seen similar issues like this one with a bad ECM. No trouble codes, right? Does the SES light come on when you turn the key on?

And, no, I have no trouble codes (other than the ones I've generated with my testing).

Replacement ECM, huh? Interesting that you said that since I have a coworker that mentioned the same thing on an old Chevy he used to have. It looks like it's above the glove box though I'm not entirely certain how it is removed. I also know the PROM chip has to be moved from one unit to the other.

So, it's possible that I have a failing ECM?

 

[1987 GMC Jimmy]

It's possible whenever nothing seems to cure a complex issue, but I'm still not satisfied on the actual MAP sensor circuit functionality pertaining to the five volt reference signal and your path to ground. Those tests are included in the link that I attached several posts ago. You've gotta hook a vacuum pump up to the sensor and probe the reference wire to see if you get the specified voltages at the specified vacuum readings. I cannot emphasize how important this is. Also, were you able to check the EGR valve to see if it can hold vacuum? You could possibly be looking at a bad ECM, but I'd really pursue these aforementioned things before I replace it. Sometimes it'll give you clues like no self-test at startup or it'll give a Code 51 PROM failure, but like I said, the fault seems to be in the MAP sensor circuit. It works unplugged, which not only tells me that the TPS circuit is good, but that removing your MAP sensor from the loop somewhat normalizes things. And if there's good ground and the voltage readings are out of spec, I'd be much more comfortable saying that the ECM is worth replacing.

 

[wczimmerman]

It's possible whenever nothing seems to cure a complex issue, but I'm still not satisfied on the actual MAP sensor circuit functionality pertaining to the five volt reference signal and your path to ground. Those tests are included in the link that I attached several posts ago. You've gotta hook a vacuum pump up to the sensor and probe the reference wire to see if you get the specified voltages at the specified vacuum readings. I cannot emphasize how important this is. Also, were you able to check the EGR valve to see if it can hold vacuum? You could possibly be looking at a bad ECM, but I'd really pursue these aforementioned things before I replace it. Sometimes it'll give you clues like no self-test at startup or it'll give a Code 51 PROM failure, but like I said, the fault seems to be in the MAP sensor circuit. It works unplugged, which not only tells me that the TPS circuit is good, but that removing your MAP sensor from the loop somewhat normalizes things. And if there's good ground and the voltage readings are out of spec, I'd be much more comfortable saying that the ECM is worth replacing.

Thanks. I will take a look at the MAP tests as soon as I can get a vacuum pump and some time to do them. I did reset the IACV today figuring that it might need it anyway with the new MAP and temp sensors this weekend. It is interesting that it starts at a much lower but smooth idle (motor warmed up already) and then increased just a bit, hunted around a couple of times for an idle, then settled into a nice steady idle. It did something similar before I changed these sensors and then went back to normal behavior shortly afterwards. So, I will drive it again on Wednesday and see if there is any difference. I'm not expecting it to do so but I thought I would share my results.

 

[TC90V10]

Surge fixed

My 90 4x4 350 1500 with 198,000 would ping like crazy, lurch at stoplights, sputter, stall when put in gear quick and wouldn't climb hills. I had it to a Chevy dealer and they changed avac line and said their tech road tested it and it's fine.

It wasn't.

I took it to the radiator ship in town that head a guy that remembered working tbi's back in the day. The tbi and intake gaskets were shot shot and he cleaned out the egr. It runs like a new truck.

And he painted the intake

Tom

 

[wczimmerman]

Got out today and replaced the distributor cap and rotor, spark plugs, air filter, and oil. I then checked the timing but the mark was all over the place. I am not sure what to make of it.

Here's the video:
https://www.dropbox.com/s/64hylrwwos7fssu/20170408_123212.mp4?dl=0