98 tahoe rough idle/miss

[kbsbkb]

I have a 1998 tahoe 4 door 4x4 with the 5.7 vortec with 108,000 miles. Lately it has developed a slightly rough idle and it also has an intermittant miss, usually when accelerating after slowing for a curve or accelerating when passing. About a month ago the check engine light illuminated and flashed which turned out to be a P0300 random misfire code which was turned off after replacing the plugs, wires, cap, and rotor. No check engine light since this was done. Also, the fuel pump and filter were replaced 9,000 miles ago. It has also had a small amount of coolant which disappears with no noticable external leakage (loses about 10-12 ounces between oil changes). I have read from various sources that intake gasket leaks could cause the rough idle and miss and also could be where the coolant is leaking. The intake gaskets have never been replaced. Could this be my problem? Any help would be appreciated.

P.S. I made a 200 mile road trip about a month ago and the tahoe missed 4-5 times during the trip. I made the same trip last weekend with no misses but the rough idle continues. Go figure!

[97cavalier]

have your coil(s) checked, a local auto parts store like auto zone checks them for free.

[sprucegoose]

Originally Posted by kbsbkb

It has also had a small amount of coolant which disappears with no noticable external leakage (loses about 10-12 ounces between oil changes). I have read from various sources that intake gasket leaks could cause the rough idle and miss and also could be where the coolant is leaking. The intake gaskets have never been replaced. Could this be my problem?

I agree, have the coils checked. But I also would be a little concerned about coolant getting in your motor. If it is losing 10-12 oz. between changes it is going somewhere. My intake gasket slowly went at about 80K, and I didn't get it fixed right away. I had very little leakage outside, but it must have been going inside. I think that was part of the demise of my rod bearing that went at 106K.

Make sure you monitor your oil level closely, and inspect it also after drainage to see if there is any coolant in it. If you pour it in a clear container it will be at the bottom, because it is heavier than oil.

[kbsbkb]

Thanks for your input everyone. I should have thought about the coil for the rough idle/miss problem. I am going to have it checked ASAP. As for the coolant problem, I changed my oil about two weeks ago, but had not removed it from my drain pan. I put it into a clear bucket last night to see if any coolant settles out, I'll post again after a few days. I have never noticed any indications of coolant in the oil when checking or changing. Also, last night I checked the underside of the oil cap and the fill tube for condensation or other moisture indications, based on a thread I found in the Suburban forum. Both were completely dry with no noticable deposits from moisture. I'm assuming based on these factors that the leak hasn't gotten too bad yet, and I am not looking forward to replacing the intake gaskets. But, based on all the problems others have posted about this issue, I am going to start collecting parts and tear into the motor as soon as practical before any major problems arises.

[kbsbkb]

I am finally going to update about my previous post. about the rough idle/miss issue, I went ahead and replaced the coil which seemed to help but did not completely eliminate the rough idle. I haven't noticed any missing since I replaced the coil. As for the coolant loss issue, I let the used oil set in a clear bucket for a week and I did notice a slight amount of moisture in the bottom. However I also remembered that I degreased and washed out my drain pan prior to changing my oil, so that may be where the moisture came from. Also, when I was changing out the coil, the engine was still hot and I could smell coolant. I checked around for an external leak, but the only possible evidence I could see was what appeared to be dampness on the front of the engine block below the lip of the intake manifold. Is this a common area for coolant leaks? I have checked the oil every day but the level is where it should be with no indication of coolant. I have lost a slight amount of coolant from the bottle since my last post. If anyone has any thoughts, please let me know.