I need your guy's help with rough idle issue

[Gordon]

Ok, 91 astro AWD... I've changed every sensor except the TPS and the main computer and it still idles rough (rough enough it shakes your brain while your sitting at a light). It will occasionally stall but only going into reverse.

Above idle it still runs great and pulls hard with no oil burn or black smoke.

I also changed the injectors, map sensor, knock sensor, coolant sensors, egr solenoid, cleaned the egr itself (wasn't really dirty).

The van only has 250,000 kms on it so this is really tickin me off.

Put a new wire set, new plugs, new cap & rotor, checked the timing, replaced all the vac lines.

Not sure if it's related but the cruis control doesn't work either but has great vacuum going to it.

I was gonna do a V8 swap but I can't get the mounts for a reasonable price or find a good set of plans.

This is our daily driver and hauls our 4 kids around so if anyone has any ideas please let me know.

[kings-x59]

I went through the same process. I went on a rehab mission on my '89. Replaced every sensor. I saw major improvement after I rebuilt the throttle body. I had to use a jeweler's screwdriver and throttlebody cleaner to scrape the carbon and dirt buildup out of the bottom of the throttle body. Particularly at the pcv hose port. I had almost no vacuum on the pcv vacuum hose before. Another ongoing problem is that the port where the pintle extends into the throttle body from the idle air control valve on my engine continually gets gunked up. The lack of vacuum on my pcv hose and valve was causing the hose on the intake side connected to the air intake to belch crud into the topside of my TBI. Thanks to the GM engineers, this happens to be right at the location of your fuel pressure regulator. When I replaced the the pressure regulator I coudn't believe that it even functioned. I was loaded with crap. My suggestion is to check vacuum hoses as a start. The rebuild kit for the throttle body is cheap, $13 bucks or so. The pressure regulator rebuild kit will shock you, I think I paid like $45.

[kings-x59]

correction: I was not loaded with crap...."it" was as in the fuel pressure regulator. Petey no snide remarks.

[Gordon]

I bought a second TBI unit and cleaned it up to better then new (muhahah die grinder)... installed a new FPR and it still doesn't make any difference to it.

I have had this van to 4 mechanic friends and they are all stumped too

[mikedamech]

Take the spark plugs out and re gap them, while there out do a compression test, 5 compression strokes each not crank until gauge stops. Also record the first compression stroke and remove all plugs with a screwdriver in throttle body. 91 Astro van, lot of miles, Timing chain, sticky valves, bad distributor cross spark plug wires. The last thing to do is the shotgun approach. If the engine shakes and stumbles off idle it sounds like a cylinder misfire not something that affects all cylinders. A sloppy chain is easy to spot, timing light moving all over the balancer, turn the crank one way stop and observe the rotor.

[Gordon]

Plugs are brand new just gapped and installed 2 days ago, same with plug wires, cap and rotor.

I was thinking maybe it was a weak cylinder so i did a compression test and all 6 are 140-145psi...pretty balanced there.

For whatever reason i decided while the engine was running to start pulling plug wires off 1 at a time... A few zaps later i found that when i took the wire off of cylinder #5... there was NO change in the engines rpm.

Any explanation as to why it would do this?

Timing chain is dead on... checked it when i changed the water pump 3 weeks ago... looks like someone already changed it for me.

I despise this engine

[GEJ]

Hi Gordon,
That is weird.I am going to tell you a case that I admit is way,way out in left field but happened.Before I found a EGR problem I had a ruff idle on my 1994 LT.We where chasing it like you and called a friend dealer Chevy service manager.He told us about a Astro that they couldn't figure out the same thing and had for two wks.After pulling the heads to redo the head gaskets,they decided to do a head job too.They put it back together and it never did it again.He said all they could figure out was they changed the valve springs and thought it was something to do with that.They too pulled spark wires while running and did a compression check having a miss on one cyclinder and not a bad compression check.They also did a cold valve spring ck and found all the springs ck'ed ok.So he thought what was going on was once the missing cyclinder got to temp it had one weak valve spring and while hot would act up.It would cool enough while trying to do the compression ck to show good.He stopped short of saying he got a service advice from GM engineering and wouldn't tell us anything like that but left us with the impression it could have been that some of the 4.3's had a batch of the same springs.

As I said this is a very strange event and a left field end result.I thought I would pass it along to you to consider.

Just for a ref:the above service manager has been one for over 15yrs.Not a newbie and was a mechanic before that.

[Gordon]

ARG... now i have a code 45... O2 sensor rich.

The stupid O2 sensor is only 3 weeks old

[k120189]

that just telling you that because of your miss fire that gas is being dumped into the exhaust and throwing a code...if the o2 is only 2 weeks old i dount its bad

[Gordon]

Man i wish I could figure out what this vans problem is

[GEJ]

Gordon,
ok it's time to take it to a dealership for a complete scan.Yes there is a difference between what you have and what they use which cost about as much as a house.Tell them to scan only and call.

[scarecrow]

did this problem ever get fixed??? i am having the same problem with my 95....

[MD/Astro\CL]

Had a similar problem with my 95'. Had just replaced the Cat and it all of a sudden started idling so rough it sounded like it would stall, and it did a few times. Took it to 3 different shops, replaced the spider, checked just about everything fuel or emissions related and still no fix. the last shop was about start looking into the valve springs and head area but figured out that the problem was the O2 and MAP sensor. Hope this helps ya some.

[blacknome]

Plugs are brand new just gapped and installed 2 days ago, same with plug wires, cap and rotor.

I was thinking maybe it was a weak cylinder so i did a compression test and all 6 are 140-145psi...pretty balanced there.

For whatever reason i decided while the engine was running to start pulling plug wires off 1 at a time... A few zaps later i found that when i took the wire off of cylinder #5... there was NO change in the engines rpm.
Any explanation as to why it would do this?

Timing chain is dead on... checked it when i changed the water pump 3 weeks ago... looks like someone already changed it for me.

I despise this engine

I think that's the answer right there. Dead plug wires are not that uncommon esp with the cheaper sets.

[BLAZER]

did this problem ever get fixed??? i am having the same problem with my 95....

95's for some odd reason had an abnormally rough idle for some reason. My best friend bought the 95 I had new from factory, had slight rough idle right off the lot. I replaced almopst everything and never did figure it out. My 95 Blazer was the same way, I've talked to others with 95's, nature of that beast for some reason.
Grodan, sounds like a plug wire or even a cracked plug ( even though they are new).