2000 suburban won’t idle

[Blue85]

The usual suspects would be dirty throttle body/IAC and leaking intake gaskets. Have you changed the intake gaskets? These engines are well known to eat them. How are the LTFT numbers? Are you getting good O2 readings? You can raise the fuel pressure at idle temporarily by disconnecting the vacuum hose on the regulator and see how the engine and LTFT responds. I think minimum pressure is 55psi, but that's key on, engine off. It is normal to drop down 7psi or so with the engine idling.

A weak MAF can screw up the fueling even though it responds normally to changes in engine condition. A simple test for a MAF sensor is to unplug it. The PCM will run speed in density (this is fine as long as you keep the throttle reasonable) and this gives you an idea if the MAF readings are wrong. If it was dead you would already have a code stored.

Also, how many of the exhaust manifold bolts are broken? Any evidence of exhaust leaks?

 

[skunked]

^ good point about the intake manifold gasket, that will cause all sorts of idle issues, more so when it's cold (had to do mine 5 or so years ago). While you have it off replace the knock sensors and knocks sensor harness with Delco replacements.

 

[bp71k5]

Shop replaced the fuel pump. Idles ok when warm, but doesn’t like to idle when first starting up. Gotta rev it a bit. Ugh...

 

[diesel4me]

Shop replaced the fuel pump. Idles ok when warm, but doesn’t like to idle when first starting up. Gotta rev it a bit. Ugh...

Could be coolant getting on the plugs due to a leaking intake gasket,they'll not fire well until the coolant burns off them..
See and white smoke or steam upon a cold start ?..
Sometimes the gasket will allow a vacuum leak until things warm up and expands and seals up more..

 

[bp71k5]

No smoke at any time. I think I’ll pick up a scanner to see if I can check what’s going on when the problem happens. Any advice on scanner choice? Prefer one that uses my phone to view sensor data unless the dedicated versions work better.

 

[Blue85]

Just go with a bluetooth interface. You can leave it plugged in all the time if you want. Something like this: https://www.amazon.com/dp/B005NLQAHS/ref=psdc_15707381_t2_B00652G4TS. So cheap you'll wonder why you didn't buy it years ago.
If you have Android, get Torque Pro ($5). I think iPhone users mostly use DashCommand. There are tons of options these days that will scan all the basic PCM parameters.

If you're interested in more, BlueDriver claims to do even more. $100 is possibly the cheapest option for getting information from modules besides the PCM (like ABS, airbag, etc.).

 

[Cili]

Have you taken the throttle body off and cleaned it?

 

[bp71k5]

Have you taken the throttle body off and cleaned it?

Nope, but that may be next. Ordered a scan tool (similar to above) as well to see if I can

 

[bp71k5]

Cleaned throttle body, air filter, still won’t idle without my foot on the gas for the first 15-30 seconds after starting even when warmed up. even then it idles at ~500rpm which seems lower than usual. Seems to have trouble idling in open loop but not so much in closed loop.

All the sensor data looks ok, except coolant temp reads a little lower than the dash gauge until it warms up. J. Feeling like a vacuum leak is maybe hiding somewhere.

 

[Fastereddie]

I have seen a misreading clt make an engine run all kinds of bad.

 

[bp71k5]

Hopefully the final update. Ended up getting the entire drivetrain steam cleaned to make working on things a bit easier. Was well worth it since the engine looks brand new now.

But I read online that a plugged PCV valve could do odd stuff to the idle and that GM offers a fixed orifice valve they use if you plug the air hole in the throttle body blade which helps prevent the valve from clogging. So I removed the PCV valve and cleaned it, reinstalled. Tried to start it up and lost power to everything. Then power comes back after 10 seconds which sounds like a circuit breaker resetting. So I check the battery cable and it’s a little loose. Tighten it up and the things starts and idles fine. Has been fine for a couple days. I’ll have to see if it lasts but seems like the PCV and/or the battery cable may have contributed.

 

[Fastereddie]

Hopefully the final update. Ended up getting the entire drivetrain steam cleaned to make working on things a bit easier. Was well worth it since the engine looks brand new now.

But I read online that a plugged PCV valve could do odd stuff to the idle and that GM offers a fixed orifice valve they use if you plug the air hole in the throttle body blade which helps prevent the valve from clogging. So I removed the PCV valve and cleaned it, reinstalled. Tried to start it up and lost power to everything. Then power comes back after 10 seconds which sounds like a circuit breaker resetting. So I check the battery cable and it’s a little loose. Tighten it up and the things starts and idles fine. Has been fine for a couple days. I’ll have to see if it lasts but seems like the PCV and/or the battery cable may have contributed.

Interesting...

 

[skunked]

My 3rd fuel pump died over the weekend in the Tahoe, go figure. Just filled the f*cker up too