Rough idle on my 90 Sub...Help! --SOLVED--

[Wheeling Arizona]

I'm still waiting for my FSM to arrive in the mail so that I can try to fix this rough idle problem. Here are the details:
<br>
<br>- The patient is a 90 3/4T Suburban. 350 FI Motor, TH400 Tranny, 74k miles.
<br>- Tends to be the worst when first starting the sub
<br>- After it warms up a bit it runs smoother
<br>- The rough idle acts as if it is going to stall out then surges back to normal...sometimes it does stall, especially if I put it in gear right away and try to drive off.
<br>- Things I have done so far to try to fix the problem are New PCV valve, New spark plugs, New plug wires, New rotor and cap, New fuel filter, Cleaned the K&N air filter.
<br>
<br>Could this be the O2 sensor of the EGR system?
<br>
<br>Thanks in advance....
<br>--Steve

 

[TopOff]

Re: Rough idle on my 90 Sub...Help!

It sounds like a cold start problem. The O2 sensor does not enter the equation until the motor and/or O2 sensor are up to temp. Some O2 sensors have built in heaters.

The best thing you can do is "ask" the computer what is wrong.

You should be able to jumper the ALDL connector and "read" the codes from the check engine light.

If you don't know how to "ask" and "read" the codes, it is listed in the Haynes manual...somewhere.

I am sure there are multiple web-sites that explain this as well, however, I don't know of any. Sorry.

BUT, I would guess a rich warm-up condition or the Idle speed circuit if at fault (dirty).

Good luck.

 

[Mr.Chevy4x4]

Re: Rough idle on my 90 Sub...Help!

Check the throttle body base gasket. These are prone to crack and cause a vacuum leak, which sounds like the problem to me.

Mike

 

[Panther]

Re: Rough idle on my 90 Sub...Help!

Can I use RTV to help seal up my TB gaskets?

And are you talking about the TB/air box gasket or the manifold/TB gasket?

 

[GMMANIAC]

Re: Rough idle on my 90 Sub...Help!

My '89 K5 does the same thing. I have narrowed it down to the ISC motor. One one occasion I cleaned it and the problem went away. If the TBI gasket fails it normally gives you a high idle condition.

 

[tomseviltwin]

Re: Rough idle on my 90 Sub...Help!

Your fuel pump could be on the way out too. My truck acted like that and ran fine for a while, but started to cut out under power and while climbing until a couple weeks later it wouldn't run at all. Just an idea.

Tom

 

[Wheeling Arizona]

Re: Rough idle on my 90 Sub...Help!

It was the fuel pump! Running good now. Thanks for the suggestions guys.

--Steve

 

[jberg62]

Re: Rough idle on my 90 Sub...Help!

I just changed mine today too.
The pump was making fluctuating high pitched whining noises and would cause the truck to surge and almost cut out.
I had to make a diagnosis and that's what I chose and
Edelbrock honored their warranty on the pump and I got it for nothing.
It's quieter now than it was when brand new.
I must've gotten a defective pump when I first got my ProFlo system.
Oh swell live and learn.

Jim

 

[4X4HIGH]

Re: Rough idle on my 90 Sub...Help!

DO NOT USE RTV anywhere on fuel items.