1991 gmc stubbling...at whitts end!

[roonie]

I have a 1991 gmc 1/2 ton with 200K on it. Its a 350 engine. The engine has been stubbling for a long time. Ive tried many things ...new fuel pump and filter...complete tune up.....checked all vacuum lines..replaced a couple lines...EGR valve.....changed head gasket and intake gasket.Checked the warm up flap inside the air cleaner. I am now very very frustrated.

This stubbling happens often but intermittant also....sometimes worse than other times like the truck is about to stall. The truck seems like its being held back like a loss of power. The truck will begin to stumble so i will pull over and rev it up and it seems to clear itself for a short while then right back to stubbling. Upon warmup in the morning she seems to run fine until hot. No codes being thrown ...service engine light NOT on. Even while at a stop i will hold the throttle part way and revs at different rpms. I am now at a loss and feel defeated. Any thoughts folks. thanks

[maxwedge]

Just a guess here try replacing the coolant temp senor, the 2 wire one.

[roonie]

Well....out of desperation i warmed up the truck......got out a can of spray seafoam and shot a pile of it into the throttle body....let it sit for 20 min...then.....ran the truck and revved her up a few times....then......on idle i continued to spray more seafoam all in and around the throttle body. Guess what....she seems to run great. I cannot believe just a simple spray job like this can improve what was thought of to be some sort of short in the elec. system or weak link in the system. This truck was bucking like a wild bronc at a rodeo show. Im sold on seafoam even if its nothing more than white gas in a can....ie. camp fuel....naptha gas. smells just like lantern fuel to me. Anyway...it worked.

[maxwedge]

Don't count on that being a permanant fix.

[roonie]

Originally Posted by maxwedge

Don't count on that being a permanant fix.

Oh....and your reasoning behind this is......?

[maxwedge]

What would pouring Seafoam into the intake do, may clean the back of the intake valves, that's about it, I have never seen this affect what you have described as being the problem you have, just being realistic coupled with 40 years in the car repair business.

[roonie]

[quote=maxwedge;7062623]What would pouring Seafoam into the intake do, may clean the back of the intake valves, that's about it, I have never seen this affect what you have described as being the problem you have,

Well maxwedge this truck is a 1991 so it should fit into your 40 years of retrology...lol. Anyway you may be correct in that the seafoam did not work. It ran great for the whole day and now stumbling again. Should i just breakdown and pay the price to have a proper diognostic done on the truck or can diognostics sometime be wrong or not accurate as well?

One thing i do recall is when i had the battleship off and changed the head gasket, i noticed the EGR passageway was plugged. I did chip it out but dont recall if it was blocked in the head as well. The EGR is testing fine. Is it just the battleship EGR passageway that plugs or the head passage opposite the battleship that may also be plugged as well. I just dont recall cleaning the head side. Sorry im 54.

[roonie]

Originally Posted by maxwedge

Just a guess here try replacing the coolant temp senor, the 2 wire one.

is there a way of testing the coolant temp sensor maxwedge?