91 blazer TBI rough idle

[Scotts91Blazer]

I recently purchased a 91 full size Blazer 350 TBI, sometimes when I come to a stop it will idle rough and occasionally stall, it also had a slight miss like a bad spark plug wire. I replaced plugs, wires, rotor, and cap... This cleared up the miss on acceleration, but didn't do anything for the rough idle. I would say 85% the truck idles properly So, I went and bought a code scanner and code book for $25, it gave me a map sensor code, as well as an oxygen sensor code (showed a rich condition). Talked to several different auto parts clerks (because they all act like they know what they're talking about, even if they don't have a clue) and they all said replace the map sensor first and see what happens because a bad map sensor will trigger an Oxygen sensor code.
Well, I replaced the map sensor, erased the codes from the ECM, and it still idles rough about 85% of the time, but I no longer get an oxygen sensor code, I just get a map sensor code. I did a little digging and found a notation that the ECM will give you this code if your vehicle idles poorly. So my question is where to look next, I did notice a slight fuel leak when I filled up on Saturday, on top of the tank, either where the fuel filler tube enters the tank, or where all the fuel lines come out of the tank I couldn't tell, but wasn't enough to spill any onto the ground. Doesn't the fuel injection system pressurize the tank, would a slight loss of pressure cause poor idleing? Any other ideas. The reason I'm thinking electronic, such as a sensor etc. is if it starts to idle rough and I turn it off the engine and restart it immediately, it clears up.

[matt67350]

IAC. IDLE AIR CONTROLL VALVE,
Sould be on the passenger side of the TBI.
Replace it and that should help.

[CHEVYJEFF]

Clean the tbi they tend to get really carboned up in the vacuum passages. Replacing the idle IAC is a good idea to. last is check egr to make sure its functioning. Use a mity vac to pull vaccum and see if the diaprham moves

[Scotts91Blazer]

I was wondering about the Idle air control valve, this is the first fuel injected vehicle I've worked on. I'm just getting brave enough to touch them, I've always tried to stick to playing with older vehicles, and taking my newer cars to my local mechanic. Sitting here wondering if I shouldn't replace both the IAC and the EGR, I'm assuming the EGR probably isn't all that different than the one on my 86 C10??? In cleaning the TBI, is there any precautions I should take to not damage it or the injectors, I'd hate to ruin anything. Any advice is greatly appreciated. Thanks for the quick replies

[HOTFUN]

I might throw one thing out that happened to my 87 TBI several years ago. I had a rough idle and it would occasionly die especially when I shifted it into reverse it would die sometimes. After three trips to the GM dealer and numerous sensors changed to no avail, they replaced the distributor shaft and it ran smooth again. They said they lose there magnetism and the spark gets irratic to casue the rough idle and dieing when it the motor is jolted changing into reverse. It wasn't that much maybe $50 for the new GM shaft plus an hour labor.

[Madbomber88]

I've been watching this thread closely. I have a 91 K5 with the same problem I think. I too replaced the plugs, wires, cap, new GM factory distributor, coil...
How much is an idle air control valve? Is that something I have to get at a dealer, or will any parts store do?

[1976K5CHALET]

Besides the tune-up....which is a good start...buy a tbi rebuild kit ($35) at napa and do what jeff mentioned as the passages underneath get clogged with carbon and the base gasket is most likely toast anyways...iac's also end up with carbon on their tips and in the bore so they don't allways idle correctly....but in retrospect i have never seen an iac fail. I'd put my money on a an egr valve....pulling up on the diaphram while running should kill the motor....and it should hold a vacum with a mitty vac....however...i've had them pass the test and still foul up...so save the trouble and buy gm if you do as the azone and checker units **** the bed all the time.

DW

[bisquik65]

Originally Posted by HOTFUN

I might throw one thing out that happened to my 87 TBI several years ago. I had a rough idle and it would occasionly die especially when I shifted it into reverse it would die sometimes. After three trips to the GM dealer and numerous sensors changed to no avail, they replaced the distributor shaft and it ran smooth again. They said they lose there magnetism and the spark gets irratic to casue the rough idle and dieing when it the motor is jolted changing into reverse. It wasn't that much maybe $50 for the new GM shaft plus an hour labor.

^^^^^YEA WHAT HE SAID ^^^^^

These dist. have problems....change it and your problem is gone.....The magnetic part will crack over time and loose spark....

[Scotts91Blazer]

Sounds almost exactly, like my problem. What I find wierd is that it will run fine most of the time, then 3 days ago it was acting up every time i pulled up to a stop sign or light, then it had been fine until tonight, and like you said, I went to back into my parking spot and put it in reverse and all of the sudden it was idling rough and wanted to stall. Hopefully in the next week, I'll finally get some time to resolve this. I appreciate all the ideas, never would have even thought of looking at the disributor shaft

[bisquik65]

One good way to tell is,look for cracks in the magnet on the top of the shaft,And when you remove the dist, turn it..if its tight turning it,the housing is wore also..If its free ,thats a good sign,just change the shaft...I think i have a known good dist here out of a 91 burb that had a 454...It ran great and i went old,hei it was a tbi 454..same dist....Let everyone know if this fixes it up...Thanks,Erick

[2002GMCGUY]

It sounds like to me that you may have a bad throttlebody to intake gasket,or possably a bad egr solinoid. The best way to check I find is to spray some carb/injector cleaner in a spray can around the base of the throttlebody.
Try that and see what happens. Chris-from Michigan

[Scotts91Blazer]

Well after numerous attempts to fix my idling problem I finally got it yesterday, thanks for all the advice, I'm now confident that 99% of my truck is in good shape at least... So I've been checking the ecm codes, and it finally spits out a code 22 or throttle position sensor voltage low code. Apparently the truck hadn't been doing it enough to record a code, but since it was starting to get worse and worse it finally stored a code. $20 later and 5 mintues to install new one and truck runs like new again. Thanks again for all the input, actually did help me eliminate a lot of possibilities. If anyones interested I bought a $25 actron scan tool from Autozone. Nothing fancy, but it also included the code book with the tool.