I am new here but the original owner of a 98 tahoe with the 5.7. SES light on, take it to the dealer and he says po125 and po300 codes. Low coolant and engine miss code at 3400 rpm, 72 mph (possible valve sticking). Never had any drivability issues, 143,000 miles on the motor.
Next he tells me he thinks it has a possible head gasket/cracked head because hydrocarbons present in coolant (following a coolant low pressure test). He also mentions an engine knock...I think that could be the cheap gas I put in it. Anyway, reading this forum on intake gasket replacements with these motors...could this be the problem or is the dealer correct and it really needs a new motor.
Other info- absolutely no visible leaks of coolant, "damp" areas on the front of the motor (under the thermostat) where (oil?,coolant?) have allowed dirt to accumulate and also on the top and inboard side of the left valve cover. Right valve cover is clean. Oil looks good, I change it fairly often and haven't noticed any coolant in it. Some trace amounts of rust-looking residue in the oil filler. This vehicle gets driven mainly on the freeway. Thanks in advance for your thoughts.

1. P0125=Engine Coolant Temp (ECT) Excessive Time To Closed Loop.

Barring any wiring problems this code indicates either the ETC sensor and/or the thermostat is faulty.

As far as the cracked head, it's possible but more likely the intake gasket.

Pull and look at all the spark plugs to see what cylinder is eating coolant.

Unscrew your oil cap. Look at the inside of the cap. Are ther any water droplets inside the cap?

Wipe the inside with a clean white paper towel. There should be no rust residue inside the cap as it is made out of plastic.

There are 2 types of intake manifold gasket leaks; internal, external or a combination of both.

Here's a good reference: http://www.market-place.com/intake/intake.htm

Thanks for the quick replies. I read the page by Suburban-97...great detailed article. First thing this morning I checked the oil cap and definitely water droplet on the cap and rust colored residue. I will now pull the plugs and hopefully post back with more info.
Assume I am looking for signs of water/coolant in the heads. What would a plug look like with this kind of problem?

OK, plugs off and they all look the same...normal as far as I can tell. Just a little light greyish-brown on the insulator...electrodes intact. No one or two look any different than the others. (Wife has the digital camera with her or I would post pics.) Are there any other diagnostic steps to verify the condition of the heads?

Thanks for the quick replies. I read the page by Suburban-97...great detailed article. First thing this morning I checked the oil cap and definitely water droplet on the cap and rust colored residue. I will now pull the plugs and hopefully post back with more info.
Assume I am looking for signs of water/coolant in the heads. What would a plug look like with this kind of problem?


If you had a water leak into the combustion chamber, the spark plug insulator/electrode would look super clean, just like if it went through a dishwasher.

Here's another good resource:

http://www.automotiveforums.com/vbulletin/showthread.php?t=441290

I will clean the plugs and reinstall...they have approx 22K miles on them. What is the required gap?

I will clean the plugs and reinstall...they have approx 22K miles on them. What is the required gap?

Look under the hood. There should be a sticker showing the spark plug gap.

OK- plugs back in...gapped to .060 in. Starts and seems to run fine. Now I have to decide if the intake manifold gasket replacement is going to fix my issue. Where can I get the gasket set? On-line?

Fel-Pro is one the most popular replacement. Good job with the plugs. The intakes would be my 1st choice as they are a very common problem. When they fail they can leak coolant into the valley below the intake manifold which gets into the internal engine and oiling system.

As far as the plugs all looking good. To me that says that a cylinder is not sucking coolant, which means to me that there is no cracked head, no cracked block. No head gasket problem.

Just My Humble Opinion

Fel Pro has the updated set if you want to do it once,made out of metal instead of plastic and Fel Pro has the best fix for the intake gaskets.The part number is MS 98000T,costs more and most parts stores sell Fel Pro gaskets.Make sure you mark the distribitor before pulling out,timing is not adjustable and the distribitor is set with a scan tool which the spec is 0 degrees.