Rough idle when cold, better when warm

Joined
Feb 12, 2011
Messages
94
Location
Columbus, OH
So something strange has been ailing my cruiser for a few days now. When I go to warm it up in the morning, it high idles properly, but when it settles down into it's warm idle, it idles rough. Once you start driving, it runs fine. Once the truck is fully warmed up it idles almost perfectly with a barely perceptible miss at idle.

I feel like there's a slight hesitation under load, but again, it's barely perceptible.

Background info:
-new plugs, wires, rotor
-new PCV and hose
-checked and subsequently duct-taped the crap out of the intake hose
-new OEM air filter
-new throttle cable
-new fuel filter
-ECM is throwing codes 21 and 28 (bad O2's) Codes have been present for the past year, truck has been running perfectly.
-VAM checks ok per FSM
-IAC checks ok per FSM

I've read through many threads, but none match my symptoms exactly. Hoping someone could provide some real time help.

Thanks in advance,

-MK
 
Last edited:
Joined
Feb 15, 2011
Messages
30
i hear ya on the rought idle..i have done the same as you under the hood...minus the duct tape..lol...i will be pulling it into the shop too swap the fuel tank shortly and i will have another go at the idle issue...i will keep you posted!!!
 
Joined
Feb 12, 2011
Messages
94
Location
Columbus, OH
Reading more in my FSM and one of the things it suggests is the fuel pressure regulator.

Anybody have any experience replacing one or think that could potentially be the cause??

-MK
 
Joined
Jun 1, 2008
Messages
158
Location
Illinois Metro St Louis
mine did that for a while and I ignored it until when it was cold it would almost die at stop lights until it warmed up. My intake hose was cracked from a filter change I did. went to O'Riellys and got a universal intake hose for 15.00 and has worked great. about 3 months now.
 
Joined
Feb 12, 2011
Messages
94
Location
Columbus, OH
Interesting that I recently replaced my airfilter as well. Although it didn't do it for about a week afterwards. I took the intake pipe off yesterday, and there were some cracks in between the bellows, however I can't tell if any of them actually went through. I did, however, duct tape the crap out of the hose it didn't get any better.

Also strange that it gets better as the truck warms up. I'm afraid to drop the cash on a new intake pipe without being confident that it will fix the problem.

I would've thought IAC or VAM, but both ohmed out correctly. Any other suggestions?

-MK
 

Mattnatti

See a Doctor and get rid of it.
Joined
Nov 11, 2011
Messages
690
Location
sOHIO
Intake hose solved my rough idle and almost stalling until complete warmup. I bought a used one from the classified section.
Night and day difference.
 
Joined
Feb 12, 2011
Messages
94
Location
Columbus, OH
So what's the likelihood that even to spite my duct tape job, my intake hose is still leaking? Do they ever tend to leak from any other place than the bellows area?

I may look into getting another one just to eliminate that variable.

-MK
 
Joined
Nov 5, 2008
Messages
296
Location
waukesha wi
My truck does the exact same thing. It runs great at the high idel speed and once at operating temp it's the time inbetween that it runs really rough and almost dies out. I will have to check out my intake pipe. If you could let us know how it runs after you change the pipe.

Thanks
 
Joined
Mar 21, 2010
Messages
478
Location
Seattle
Just for kicks, unplug your O2 sensors and see if it alleviates your rough idle issue. When you start your engine it is running on a predetermined value set programmed into the ECU. When you reach operating temp, your ECU starts to rely on feedback sensors like your O2 sensors to monitor and adjust your air fuel mixture. If your O2 sensors are sending intermittent or erroneous readings it could cause a rough idle, but typically bad O2 sensors result in even more terrible fuel economy and not a rough idle. If you have an FSM there are some components that could be immediately affecting your idle that should be checked per the FSM:
VAF
Idle air control motor
Pair system since you have a 94.
 
Joined
Nov 5, 2008
Messages
296
Location
waukesha wi
I will try that on mine and see what happens. I had a major leak behind my converts and that was causing some over fuleing issues. I fixed that and replaced my stock muffler with one that breathes better. which seemed to help fuel ECON a bit. I'll check out the 02 sensors
 
Joined
Feb 12, 2011
Messages
94
Location
Columbus, OH
I cleaned the IAC, but it looked pretty nice. Sprayed some TB cleaner in the TB and used a rag to clean where the throttle plate seals to the TB. I also disconnected the O2's since they were fuct anyways.

What I DID find was that the breather hose running from the valve cover right where it says "24 VALVE" to the top of the TB was split. I'm guessing that could cause some issues. I taped it up, and fired the truck up. It still has a little roughness to the idle, but it appears to be marginally better. Doesn't feel like it's going to die any more. So I'm going to replace the breather hose for sure, and if that doesn't fix it altogether, I may go ahead and replace the VAM to TB hose.

-MK
 
Joined
May 22, 2007
Messages
172
Location
Seattle, WA
MKTSC said:
Reading more in my FSM and one of the things it suggests is the fuel pressure regulator.

Anybody have any experience replacing one or think that could potentially be the cause??

-MK

I have the same issue with my 97. Replaced all the same things and then tried the FPR.

Only change was that my wallet was lighter after.....
 
Joined
Feb 12, 2011
Messages
94
Location
Columbus, OH
My coolant gauge has worked intermittently for the year that I've owned my truck. It'll go weeks and be just fine, then weeks where it won't work at all. The thing is, the truck's always run fine even to spite this.

I replaced my old breather hose and intake hose, and the problem still exists, although not as bad. It only does it when the engine is warm enough not to high idle, but not fully warm. Once it's fully warm, it runs perfect. I was thinking I had a binding IAC, but you think it's coolant sensor?

-MK
 

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