Surging Idle, And......?

[Rock-N-Blues]

Hey There Crazy Guys & Gals!
While my 91 Astro with TBI starts right up and runs/drives in a manner that would be acceptable to many, there are a few issues. When I first got it, there were a bunch of little running issues, wonky shifting, etc. I've done quite a few simple things to rule out the obvious stuff but still a few things remain. I'll give a list of what already has ben done and then a list of details of the problems.

This whole list was done all at once, so I'm not sure what fixed what, but everything is much improved! Shifting is now just about what I would expect and overall driveability is much better. So far I Have:

A) Oil and all filters changed/replaced.
B) New plugs, cap, rotor and wires. Plugs were all fairly even/similar, with two just a tad darker than the others. None were wet or oily. No noticeable misfires or apparent engine skipping.
C) All vacuum hoses replaced and firm connections confirmed, to rule out a hidden leak. The hose to the PCV valve was broken right off and crumbly. I suspect this was at the root of the wonky shifting issue, as well as some of the other issues. I do suspect that most/all problems are vacuum related, as in something leaking or in the sensing of vacuum, "Somehow, Somewhere, Someway," a cool Kenny Wayne Shepherd song, BTW!
D) Put in two full bottles of Seafoam into about 1/2 tank of fuel (for an estimated ratio of +2 ounces of Seafoam per gallon), drove for about 40-50 miles of varied conditions, stomping on it, etc, then topped off the tank and have driven another couple hundred miles. Man 'O' Man, that Seafoam is some kinda magic potion!

Now here's a detailed description of the maladies, bearing in mind that the issues become more pronounced when the AC is on. I do live in Las Vegas, so AC is kind of a necessity! Also, all the same problems were much more dramatic before doing all of the above, but still exist, to some degree.

1) Upon cold startup, it runs well, but exhibits a somewhat random surging of idle speed. Not really drastic, but noticeable and it mostly clears up as the engine reaches normal operating temperature.
2) With light to medium acceleration from a stop, after each "up shift" it seems to bog for a second, +/-, and then picks right back up. There is no noticeable engine skip/misfire, just a slight hesitation/bog. It is more apparent/dramatic on the 1 to 2 shift than on the 2 to 3 shift. Since doing all the above steps, it is almost non-existent with the AC off.
3) When stopped at a light, in gear, it runs as smoothly as these engines can, but then "hiccups" followed by a short surge and then runs smooth again. Before performing the above stuff, it would happen at shorter intervals, but now it needs to be a fairly long light for it to happen.
4) Driving at a steady speed, either 30-45 in non-overdrive or on the highway, I feel the engine randomly and slightly bogging and then picking up, with intervals ranging from probably 1/4 second to 2 seconds. Upon acceleration, this mostly goes away. This no longer happens at all with the AC off, but still occurs with the AC on and it feels exactly the same as if I cycle the AC on and off at the controls. I know that some vehicles have some sort of sensor/AC compressor switching system that turns off the compressor when vacuum drops under acceleration, but am not sure if that type of system is on my truck, or what the parts are that operate such.

Please understand that I have no problem spending $$$ on anything that actually needs to be done, but am not well off enough to simply, randomly and willy-nilly, "throw parts" at this van until the problem goes away. I'd be much happier to go at it piece by piece and in the correct and most efficient order, in hopes of not only fixing my issues, but also learning and understanding what each part does and how to recognize each part's failure in the future.

Thanks Crazy Guys & Gals,
Gene

[Leeann_93]

Time to pull and clean your IAC and EGR valve.

Somewhere on here there's a way to test your TPS sensor as well - it's a potentiometer. I just replace them, personally. Same with MAP sensors.

Also, coolant temp sensors (not the sender for the gauge, the sensor for the computer) can cause all kinds of funky running issues.

[Rock-N-Blues]

Where do I find the IAC and how do I clean it? Also, where do I find the coolant temp sensor? I already know where the gauge sender is.

Let me make an assumption here and correct me if I'm wrong. It would seem that any of the original emissions, injection, and sensors parts that are functioning properly are better left in place, as opposed to simply replacing for the sake of replacement and/or troubleshooting. Yay or Nay?

Also, I'm gonna guess that even though the van may end up running and performing better with a blocked off EGR, it won't likely pass a real smog test that way?

Thanks,
Gene

[Mmusicman]

Bucking or erratic operation at "part throttle" can be caused by a worn distributor. I has happened to me, as well as others. Idle will be fine, full throttle will be fine... part throttle will have issues. It can even feel like the transmission is at fault. New distro and my problem was solved. Seems they are good for about 150K.. on average.

You can pull the cap and check for play.
Not saying this is your problem.. just saying it "could" be.

There have been numerous post on this.
Here's the most recent guy that had this problem:
viewtopic.php?f=70&t=122985

[Leeann_93]

IAC is on the side of the throttle body.

Coolant temp sender is in the passenger-side head.

[Rock-N-Blues]

It seems that most folks that fix their problem with a distributor replacement have the pancake type. Did the earlier "tower" type, like this one, have the same problems?

9b444c4098c6940ee63c2414e6898b14.jpg (28.33 KiB) Viewed 1840 times

Oh, and FWIW, I checked the distributor shaft for play. The rotor does not have much "rotational" play or side to side play, but does move vertically up and down a fair amount, maybe 1/8" to 1/4".
Thanks Folks,
Gene

[Coaster]

the early ones dont have the same issues. i would be looking at exhaust leaks before the o2

[Rock-N-Blues]

No visible leaks and quiet as a church mouse even under a load. I'm in the dry Las Vegas desert and exhaust system do not rust and have very few failures, unless disassembled, messed with and put back together improperly. I have occasional access to a lift and will give it a thorough inspection as soon as is practical!

With an OBD-1 system, might I find any codes that may be helpful? Is there a "poor man's" way of reading the codes, like on the earlier Fords, with a jumper wire and test light and counting the flashes?

FWIW, I just ordered a Skip White distributor to put that possible variable to rest, but expect little to no satisfaction on that front, given only 80,000 miles on the truck! It did, however, already provide the accomplishment of lightening my wallet by $57-ish!

Thanks,
Gene

[Coaster]

no, no useful codes. i have obd1. the symptom you describe is +2 degrees advanced dist but i am sure that is just the surface rather than the heart of the problem. something else is underlying. could be as simple as the ECT or EGR. i've been though my engine chasing a surge for 4 years. it was timing, exhaust leak, incorrect o2 sensor. i had a severe no start that was the CPI harness connector and that doesnt apply to you. these engines are simple, but diagnosis is hard due to CPU and weird leaks and wear items

[Rock-N-Blues]

Ahhhh, Sooo,
It seems to boil down to a choice of either chasing my tail down into a rabbit hole of guessful/hopeful parts replacements or just dealing with little, minor annoyances and simply fixing real problems as they arise? I will obviously replace any items that are clearly failing, but it just doesn't seem logical to replace multiple parts in the hopes that they were the actual culprits! $40 here, $65 there and $29 over there all adds up pretty quickly and provides little satisfaction when the problem remains! On the other hand, proper cleaning, testing of EGR, IAC, along with replacement if necessary, as well as the thought of testing/replacing a 26 year old TPS is well within reason. I'll keep ya all posted on my progress.
Thanks,
Gene

[Leeann_93]

Yep, a paper clip and a pen and paper and you’ve got the codes, but they’re a bit less helpful.

http://www.troublecodes.net/gm/

[Rock-N-Blues]

Yeah,
I now see how to get the codes, but at that link, they don't show a listing for a 1991 VIN Z. They show up to 90 for a VIN Z and one might assume it's the same for 91, but......? About the only code that might seem useful is the one for EGR, but even then?
Thanks,
Gene

[AstroWill]

With an OBD-1 system, might I find any codes that may be helpful? Is there a "poor man's" way of reading the codes, like on the earlier Fords, with a jumper wire and test light and counting the flashes?

Paperclip works to get the stored codes, but if you want what I call pseudo-live data it will cost quite a bit more.