Low idle, stalling- 1988 Chevy K2500 4x4 w/ 350

[DonA]

We've been having a problem with my son's 1988 Chevy K2500 4x4 w/ 350 for the past couple of weeks.

The idle speed is low- 500rpm or so, and erratic, it hesitates when you get on the gas, sometimes stalls after reving and almost stalls when put in gear. This often happens when the truck is cold though it's worse when it's warmed up.

So far we have-
-Checked and set the timing
-Replaced the EGR valve- the old one failed a vacuum test
-Tested the EGR solenoid- it appears to be fine
-Checked for vacuum leaks and found none
-Tested the IAC- it moves and the resistance on my DMM is correct
-Tested the TPS- it appears to be fine
-Tested the CTS- it appears to be fine
-Tested the MAP- it appears to be fine
-Fuel pressure is good
-Fuel flow is good and the filter is recent
-The spark plugs are all tan and in good shape

I'm a little wary of the new EGR valve. It's a Duralast and I've read that we should've stuck with an OEM unit.

The iginition module was replaced recently due to another issue.

We replaced the engine in this truck with a rebuilt short block about 8,000 miles ago and it has run well until recently.

Thanks in advance!

 

[SkyHighColorado]

Sounds like an IAC issue to me. Clean up the whole tb really well make sure all the vaccum veins are nice and clean in the tb. I would replace iac....that's what it sounds like to me, but I may be wrong. ..

 

[K1500 gramnolds]

My truck had this problem until recently. When I replaced the IAC, I cleaned my TBI unit as best I could, replaced the base gasket, and re-installed the unit. Problem was a little better but not solved 100%. This last week I had to redo my intake gaskets, so the TBI came off again, was disassembled, and was cleaned again along with the intake passages being cleaned up, EGR removed and soot cleaned out, ect. After all was back together truck runs WAY stronger. Not sure if it was a combo of vacuum issues w/ the intake gaskets and a dirty TBI unit tho. It actually seems to me it idles a bit high now when in park but when I drive it, it doesnt stall at the light and is in a good rpm range so I`m happy.

 

[DonA]

Thanks!

Last night my son disconnected the EGR valve and plugged the lines going to it and the EGR solenoid and the truck ran much better. I think we're going to try another EGR valve. He has it blanked off with a gasket for the time being and it's running well.

Also, when we installed the motor we did not take the TBI off of the intake manifold and replace the gasket. I think we'll do that next. We'll clean up the TBI while we have it off.

 

[K1500 gramnolds]

I know for a fact my egr isn`t working properly with all of the soot stuff I cleaned out, so essentially mine is blocked off too. Any air leaks be it base gasket on the TBI or elsewhere will cause erratic idle and vacuum problems. If the other areas are good, ensuring the TBI base gasket is good too is just another piece of the puzzle.

 

[DonA]

We rebuilt the TBI last night and the truck is funning the same- the idle is low, unstable and the truck feels like it wants to stall- better than when I started this thread, not as good as we'd like it to be.

We're supposed to tow his racecar to the track with it on Sunday and we're not overly confident.

 

[SkyHighColorado]

When you say rebuilt does that mean you put a brand new IAC in it? If not you need one and a new egr.

 

[DonA]

We did not replace the IAC. It still might be the problem.