1999-7.4L rough cold idle, then high idle when warm

[monroeta]

For the past few months, I've been having a realy rough idle problem. At stop lights, it sounded like a race car idling, then when the light turns green and I hit the gas, it sputters a bit and finally takes off - running fine while moving. After a while, the idling got so bad and I never knew how long it would be before it finally got going when I stepped on the gas, I started playing a bit with the sensors. The scan codes mentioned a 0174 - running rich - I tried disconnecting the IAC. That didn't seem to help. Finally, I found that when I disconnected the MAF, the idle stayed up around 1800, but at least it ran smoothly and it was drivable.

 

Finally I got a chance to get it to the garage. They ran through the diagnoses and determined that the MAF sensor was bad. I reitterated to them that it had been disconnected for a while up until the morning I dropped it off. They stuck by their story saying the sensor is bad. When they told me the cost of the part, I thanked them and put in a new OEM version of the MAF sensor. Hooked everything back up and I'm back to where I started - very rough idle. I had the codes checked again and sure enough, it points out the MAF sensor as well as running rich. Looking back at the service manual, I noticed that it will retain the code for the MAF sensor even after it's fixed until the engine has gone through 40 complete warmup cycles. Now I'm wondering if the MAF sensor ever needed to be replaced.

 

Anyway, I stll have the same symptoms. When I first start it up, it's very rough - the idle will bounce high for a bit a few times like it's trying to find the right mixture. If I just leave it sitting in park, after about 10 minutes when it starts warming up (probably entering closed loop mode), the idle goes up to 1800 to 2000, but it smooth (just like when I ran with the MAF sensor disconnected.) When it first goes to the higher idle, something smells funny for an instant, like something got real hot, but don't see anything in the exhaust. I did find that if I run it with the MAP sensor disconnected, the idle stays down aound the 700 range like it should, but it still idles very rough.

 

Driving into work (about 20 minutes highway), when I stop at lights, it seems to idle smoothly now, but up around 900 rpm in drive. However, when I parked it, I noticed the idle went right back up to 2000 rpm when I put it in park.

 

By the way, the person that checked the codes for me also checked the fuel pressure and everything looked good there. He suggested some sort of vacum leak - maybe a leak in the mid intake plenum gasket. I did have the intake manifold gaskets replaced last summer (about 9 months ago, by the same place that said it's the MAF sensor.) That also seems to show up a lot when I see similar types of problems in online forums. However, in other forums, I never found anyone mentioning the idle going up after the engine warms up.

 

Any suggestions? I'm trying to figure it out on a limited budget (which is why I put the OEM MAF sensor in myself).

[monroeta]

I said earlier that the smell for an instant when the idle goes from low/rough to high and smooth (while sitting in park) smelled like something hot. Change that to a smell like when you first strike a match.

[Carder09]

Did you try cleaning all and any components like throttle body and such, hell you could clean the new MAF sensor if you wanted but it sounds as though it could be an ecm issue also. Other than that I'm partially at a loss.

[jaredlucky16]

174 is a lean code not rich. I would look for a vacuum leak but mainly suspect the upper intake gaskets. spray around with carb cleaner and listen for a change in the idle or monitor O2 sensors with a scan tool watch them stay lean around 100mV then when you hit the leak the voltage should go up to around 900 mV

[monroeta]

Oops, you're right, I said rich while thinking lean. I will try the carb cleaner test and see what I can find. I've seen the same suggestion in other rough idling problems - though they usually say to use Nitrus. Thanks.

 

Also, not sure how this might relate, but it was raining all day yesterday and while I was driving home, I noticed that it seemed like it was engaging the torque converter as low as 35MPH (I was only going up to around 40MPH at the time.) Normally, it won't engage until I'm over 60MPH. This morning when things were a bit dryer, it wasn't doing it anymore. Wierd.

[monroeta]

Did the test with the carb cleaner yesterday and found that it is able to suck it in just about all the way around the upper intake gasket. About a year ago, my garage replaced the intake manifold gasket because of a coolant leak. I have to look back at the invoice to see if they replaced the upper intake gasket too, but I was wondering if it would be advisable to try retorqueing the bolts or if I should have it taken apart and the gasket replaced (possibly again).

[monroeta]

By the way, Thank you to both "enthusiasts" that gave suggestions. Related to my question about retorquing, I'm guessing it's best to go through taking it apart and putting it back together since the bolts have locking compound on them (or at least are supposed to have) that would change the torque setting now that the compound would be dried. I still welcome you're opinions.

 

I did find the invoice and it says they replaced the lower and upper intake manifold gaskets - so I will probably be talking with that garage to see what they offer to do for it .

[monroeta]

FIXED! I left the truck a couple days ago at my garage and made it clear that I wanted them to check the intake manifold for vacuum leaks. They did find that the upper gaskets that they replace just under a year ago were failing and causing the leak. They had used OEM gaskets - presumably because they couldn't get the GM gaskets at the time - and found that the OEM gaskets don't hold up in this engine.

 

I remember seeing a few other people with similar problems in other forums indicating that OEM gaskets are not a good idea for GM trucks - one of the few exception parts when comparing OEM to GM. My GM garage told me they are changing their policy to never use OEM intake gaskets on the 7.4L engines, only GM gaskets.

 

FYI, The service manager brought up the fact that they did the job less than a year ago so they replaced the upper gaskets at no charge to me.

 

I wanted to thank you for helping me diagnose the problem.

 

-Tod-

[jaredlucky16]

No problem glad its fixed