1997 Bravada - Slight Rough Idle, runs great - HELP - Blazer Forum - Chevy Blazer Forums

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Old 03-06-2010, 10:29 PM
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Default 1997 Bravada - Slight Rough Idle, runs great - HELP

Hey Guys - Need some assistance with a tough one.

1997 Bravada
165k, seems to have been taken care of. Original owners.
Recently had plugs and wires replaced as well as fuel pump. Filter was replaced too.
Came to me for rough idle and lack of power at times (lack of power that it can barely move). Also excessive fuel consumption?
Ok, so my first thoughts are clean the throttle, IAC and MAF - check. Throttle was filthy - didn't remove IAC when I cleaned.
Had 3 codes - 2 lean DTCs and a P0300. I didn't write them down cause the check engine light wasn't on when the truck came to me nor did the owner mention it being on.
My thoughts were that the lean DTCs were from the fuel pump going out along with the P0300 that may have set and the plugs/wires were replaced and nobody cleared the codes.
So I got it cleaned up but the idle seems a little rough in drive and reverse only. It idles nice in Park and N.
I drive the **** out of it, no problems, no lack of power, just the AWD shake these things are known for.
Well the engine wasn't getting hotter than 170 degrees, so I changed the t-stat now we got 200. Cool, there the fuel consumption.
No change in the idle.
I pull the EGR off and it looks fine and the pintle is seating all the way, but I clean it anyways. I verify the pintle holds with the carb cleaner in the valve and check for leaking at the pintle.
No change.
I sit down with the scan tool and start looking at stuff.
I see the IAC is being commanded to 0 and it's position is 0 as well. Also, the computer wants the truck to idle at 600 RPM but it won't go lower than 675RPM.
I command the EGR and it idles rough at 20% and 30% wants to die.
I command the Purge to 100% and the idle smooths out. I command to 0% and it runs like crap for a minute while it adjusts.
I check for vacuum leaks and find a few marginal hoses, replace them and the same thing.
It idles nice but I think it should idle smoother and better and the IAC should not be at 0.
When I put it in drive the desired idle is 550RPM but the engine still won't go lower than 650RPM.
As I ran it tonight, I think the fan clutch may be sticking. But that shouldn't cause this symptom but may also be contributing to the excessive fuel consumption.

It looks like a vacuum leak but I can't find anything.
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Old 03-07-2010, 08:25 AM
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You have a vacuum leak somewhere.
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Old 03-07-2010, 09:00 AM
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A few possibilities for you to check.

First thing, the IAC must be removed for proper cleaning. It should also be moving off of zero and making lots of adjustments as the RPM's change. If it doesn't then it may be stuck in place due to deposits or simply a frozen stepper motor. Both are common on these engines. When the key is first turned it should cycle from full open to full closed on most makes/models.

Test your TPS with an OHM meter. You may find the exact procedure and proper values floating around this board somewhere but it is also in your Chilton manual.

Check the PCV valve. if it's not opening and closing properly then it can act like a vac leak. It should rattle easily when you take it out.

Check for exhaust restrictions such as rattles in the cat. A partially plugged exhaust can cause a lot of different problems including many that can mimic a vac leak such as you have.
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