Rough idle when cold

Bubba14 · January 19, 2004

My 99 silverado has a 4.8 liter with 89k miles.

Problem is when engine is cold it will hardly idle. rpm's go from 300-1100

The colder the weather the worse it is.

After the engine warms up to normal operating temperature it runs fine.

Here is a list of what I have done,

Had the fuel injectors cleaned two times, (once by chevy service dept),

added dry gas to two tanks of gas, changed spark plugs, and wires, cleaned throttle body real good, changed fuel and air filters, checked vacuum lines, sprayed carb cleaner around base of throttle body, intake, vacuum lines, etc. and did not change the idle rpm's.

Cleaned egr, changed idle air sensor, checked fuel pressure regulator by removing vacuum line (no gas came out).

The service engine soon light came on and scan indicated 171 and 174 too lean bank one and too lean bank two. Had them reset and have not come back on.

Any tips or help is really appreciated and Thanks in advance.

Butch 02 Denali · January 19, 2004

Bubba,

Your MAFS has a temp sensor in it. It could be dirty or bad. If you have a buddy with a simular truck, swap mafs and see if that cures it. You can also get a new Z06 Mafs on line for $109 from GM Parts Direct. If that doesn't cure it I would change the coolant temp sensor in the block. 90% chance it's one of those two things.

Butch 02 Sierra Denali

Bpot · January 19, 2004

INTAKE MANIFOLD IS LEAKING AIR, HAPPENS ALL THE TIME, IF YOU HAVE A SCAN TOOL TO WATCH FUEL TRIM YOU CAN SPRAY SUPER LUBE AROUND THE GASKET AND WATCH YOUR VALUES MOVE TO MAKE SURE.

JayMan · January 19, 2004

A plugged IAC passage will cause cold-start problems, including poor or no idle. If you've taken the TB and IAC apart to clean them, it is VERY easy to get a air (Vacuum) leak.

Especially the IAC motor. There is an o-ring that is usually stuck at the bottom of the IAC well, so when you yank out the IAC motor, the o-ring is lost in transit or damaged.

Bpot has the correct diagnostic tip here. The normal rich-lean-rich-lean flip/flop will not behave correctly.

By chance, did the GM dealer or whoever else have any reason to actually REMOVE a fuel injector? There is a caution in the Helm that you must be very careful removing the injectors: the tiny o-ring can stay behind and normal practise is to put new o-rings on. Instant manifold leak.

Bubba14 · January 20, 2004

Well it sounds like you guys are pretty sure its a vacuum leak.

I am going to go back and recheck everything again since vacuum leaks are difficult to find.

I did not remove the throttle body for cleaning. I cleaned it as best I could in place. I cleaned it with the IAC sensor removed to ensure I got the passage clean or at least hope I got it clean. I did reuse the o ring on the IAC and I coated it with a light film of oil. Visually the o ring looked fine.

No fuel injectors have been removed, Jiffy lube cleaned them once with their fuel injector line of bull**** to take your money. It ran so bad after they got through with it I then took it to the Chevy service dept and they cleaned them. This helped a little but not much. This was in November.

Oh by the way last night on the way home from work the SES light came on again.

Your help is really appreciated, Thanks Bubba14

JayMan · January 21, 2004

Well it sounds like you guys are pretty sure its a vacuum leak.
I am going to go back and recheck everything again since vacuum leaks are difficult to find.

I did not remove the throttle body for cleaning. I cleaned it as best I could in place. I cleaned it with the IAC sensor removed to ensure I got the passage clean or at least hope I got it clean. I did reuse the o ring on the IAC and I coated it with a light film of oil. Visually the o ring looked fine.

No fuel injectors have been removed, Jiffy lube cleaned them once with their fuel injector line of bull**** to take your money. It ran so bad after they got through with it I then took it to the Chevy service dept and they cleaned them. This helped a little but not much. This was in November.

Oh by the way last night on the way home from work the SES light came on again.

Your help is really appreciated, Thanks Bubba14

Yeesh, Jiffy Lube. That might explain it.

How exactly did they do the procedure? A MotorVac system properly hooked up? A pressurized can like the 3M system? I would worry that if they used an improper procedure or - this makes me shudder - a DIRTY system to "clean" your injectors, that dirt might have been circulated through your fuel rail.

Every injector has a very fine mesh screen at the top as a last line of defense to keep floaters and chunks from plugging the solenoid. It's possible most or all of your injectors are clogged at the inlet or something has been knocked loose and the tips are clogged.

Since most of the injectors on the Vortec are fairly easy to remove, I would try to find a reputable fuel injection shop - maybe a commercial HD truck shop like a Cummins engine rebuilder might have the proper tools - and have at least one of the injectors bench tested. Maybe if the price is right, have ALL of them bench tested.

The bench testing involves hooking the injector up to power and applying a safe solvent or working fluid of the proper pressure. You can then visually observe the spray pattern to see if the injector is obviously plugged.

Although the IAC motor is a possible vacuum leak source, geez I really doubt it would leak THAT bad. Still, wouldn't hurt to get a new o-ring and try replacing.

Just a thought. It's not unheard of for the fuel injection "cleaning disaster" to happen.

Bubba14 · January 28, 2004

Ok I admit taking the truck to Jiffy Lube wasn't the smartest thing in the world.

It was already running bad before I let them tinker with it. They just made things a little worse. Jiffy lube was the first on a long list of my failures with repairs.

Now after these posts I have tried several different cures. So far none have helped much. I don't mean that in a negative way. Your help on this forum is really appreciated.

I have tried to find a vacuum leak. I have nearly drowned the engine with carb cleaner. I have sprayed every vacuum line, every gasket seam on top of the engine, every thing in sight, but the engine doesn't respond.

anybody got a match

JayMan · January 28, 2004

Ok back to basics:

You say you already changed the plugs and wires. Did you use the OEM wires and plugs. The CPC system is "up integrated" and can sense resistance changes, that' s how it can detect misfire for OBD2 compliance: the values are based on Delphi's stock wires and AC Delco stock plugs. Using the wrong plugs and wires might confuse the CPC.

The IAC might be acting up, but you say you already cleaned it. The Helm mentions the proper test procedure.

Did you check fuel pressure? On the passenger side of the block (Facing the motor from outside, that would be your left side) the fuel rail has a Schraeder valve, just like the valve on a tire.

Pick up a fuel pressure gage at NAPA or Autozone or whatever. With the motor off, attach the gage and wipe up any spilled fuel. Watching the gage, have a "helper" turn the key to "ON."

The gage should quickly rise to normal pressure, about 56-64 psi. Have your helper start the motor and verify the fuel pressure is still ok.

Otherwise, it could be either ignition or something really bad with the fuel injection. This will require detailed diagnostics, and you really need some sort of scan tool that will let you look at O2 operation and injector pulsing. The Autotap is a reasonable and fairly detailed system.

Bpot · January 29, 2004

Just get the intake manifold gaskets replaced, If you want I will bet money on it thats what it is, I just seen 2 more today just like that, they always have 2 codes in the computer 171 and 174. It only happens when cold because the gasket is small, when it heats up it swells. Shouls not cost much (bout 2 hour labour) plus gaskets.