89 S-10 Blazer 4.3 Rough Acceleration

[89blazer]

I just bought a 1989 S-10 Blazer 4.3 V6 Auto with only 100K miles on it. I test drove it cold and it was decent. Once it's warm though, the acceleration and idle are very rough while in drive. Engine idles very smooth in Park & Neutral and revs smoothly. I checked the trans fluid and it's way over filled but clean and fairly odorless. Would being over filled cause the problem or is it over filled to hide a worse problem? Does it need a new tranny? Is this typical of these? I spoke to a friend with a 91 Astro van with a 4.3 and he's gone through 4 trannys already. Argh! Thanks in advance...

[Mikado14]

IMHO, your comparing an apple to an orange as far as the Blazer and the Astro Van are concerned.

As to the overfilled tranny, if you see no leaks or you don't have evidence of oil from the tranny in the antifreeze, than the person just overfilled it. Wouldn't be the first time I have seen that one.

Now, about the rough idle. Do you have any codes or is the MIL lighting up?

Did you do a tune-up since you purchased this vehicle? When was the last time the previous owner did a tune-up?

I would do a tune-up first. Plugs, wires, cap and rotor, fuel filter, air filter and PCV.

[89blazer]

Thanks for your input, Mikado (not sure what IMHO means). The seller just tuned it up with plugs, wires, cap & rotor. He said the plugs were nice and gray, all the same color. I'm pretty sure it's not the engine because it runs fine in neutral and park and revs real smooth. But when it's warm, putting it in drive makes it run rough and accelerate rough. When on the freeway it runs smooth. Just at idle and acceleration it's rough. I don't know what the MIL is and where would I see codes?

[Mikado14]

IMHO = In My Humble Opinion

Here's a few things you can check.

EGR, remove and clean. Your EGR should be vacuum actuated and the diaphragm may be ruptured. To check, unplug the vacuum line and plug the vacuum hose with something that will fit, a sharpened pencil will work. Start the car up and see if there is a difference.
Air Filter, replace
Fuel filter, replace
Fuel pressure, check - If you don't have the guage, ask around with some of your friends.

The MIL is the "check engine light" it stands for "Malfunction Indicator Lamp". If this lights up, the computer has detected a fault and has stored a code. You will need a code reader to retrieve it. If you are close to an AutoZone, they perform this for free. And they may be able to check your fuel pressure as well.

[89blazer]

Great, thanks!

[OverBoardProject]

The Oxygen Sensor is often overlooked, and could easily cause the problems you mentioned making the computer think that it's running lean, or rich

[Mikado14]

If the O2 sensor was at fault, there would be a code and the MIL would be on. You are correct in that a bad O2 may not show up until the engine is warm and the ECM goes to a closed loop condition but then again, a light would be on and he has not answered with that info.

[89blazer]

Thanks guys. I've been able to duplicate the vibration in Park so it's definately not the tranny. If I rev to 1500 RPM and hold, I can feel kind of an out of balance type harmonic vibration in my seat. I think maybe the exhaust pipe or muffler is touching the floorboard or I have a broken motor mount.

[89blazer]

Okay, I disconnected the vacuum line from the EGR valve and the roughness still exists once warmed up. No change. However, there was no vacuum present on the line when I disconnected it. This has an EGR solenoid in-line between the throttle-body and the EGR valve. How would I verify that's not the problem without replacing it? There's good vacuum on the feed line to the solenoid but the guys at the auto supply said it's impossible to test in the driveway. It only comes into play under certain driving conditions. Also, how do I tell if the diaphragm is bad on the EGR valve once I remove it...see if it will hold a vacuum on my tongue? I just thought of this -- my temp gauge shows 200-210 during normal operating conditions. Isn't this a little hot? Would you recommend a cooler thermostat, or none at all (this truck runs great when cold).

[quaddriver]

Originally Posted by 89blazer

Okay, I disconnected the vacuum line from the EGR valve and the roughness still exists once warmed up. No change. However, there was no vacuum present on the line when I disconnected it. This has an EGR solenoid in-line between the throttle-body and the EGR valve. How would I verify that's not the problem without replacing it? There's good vacuum on the feed line to the solenoid but the guys at the auto supply said it's impossible to test in the driveway. It only comes into play under certain driving conditions. Also, how do I tell if the diaphragm is bad on the EGR valve once I remove it...see if it will hold a vacuum on my tongue? I just thought of this -- my temp gauge shows 200-210 during normal operating conditions. Isn't this a little hot? Would you recommend a cooler thermostat, or none at all (this truck runs great when cold).

the guys at the auto supply are weenies.

get a vacuum gauge, hook up to the EGR side of the solenoid, then when warm, slowly increase the rpms in neutral. the solenoid will open after 1500 rpm and it you nail it past 75% the solenoid will close again.

that will prove the solenoid.

next use a vacuum pump (with gauge) hooked to the egr and pull some vacuum. it should hold for at least 5 seconds before bleeding down (it actually should hold for hours, but slow leaks can be tolerated)

[BlazerLT]

BTW, have you changed the fuel filter lately?

Sounds like it is starved for fuel.

[89blazer]

Replaced the fuel filter, put a 160 thermostat in, cleaned the EGR valve (it was hardly carboned up and held vacuum indefinately), I wasn't getting good readings on the EGR solenoid so replaced it, flipped the air cleaner cover over for more air -- no difference :-/. Next items: change PCV valve, check fuel pump pressure, check spark plug gap. What else would change once warm and cause it to bog from idle to 2000 RPM? I'm also thinking clogged cat (anyone know where to get a cheap cat?). Any other ideas?

[BlazerLT]

160 stat will do it.

180 is the lowest you should ever go.

[89blazer]

The reason I'm trying the 160 stat is because it runs like new when cold, nice and smooth with good power. As it heats up, the idle gets rough, near stalling, and mild acceleration is rough and labored with a vibration. If I stomp on it or get over 2K, it runs smooth. I was convinced it was EGR related. This is frustrating as I spend most of my time in traffic under 30 mph when it's most prominent.

[BlazerLT]

remove the 160.

It is not allowing the engine to get up to temperature and probably not allowing your engine to go into closed loop.