Have an '03 Z71 Suburban, 5.3L with approximately 106,000 miles. I've also had a mystery coolant leak for about the last 20,000 miles or so.

Less than 6 months ago, I replaced the head gaskets and had the heads machined and bolted back on hoping that would end my problem. Less than a month later the problem came back, but now it seems worse.

Coolant is still leaking, but now my oil pressure fluctuates wildly between 20 and 40 and stays near 20 at highway speeds about 1000 miles into an oil change (even with heavy oil). Adding a quart fixes it for awhile before it happens again.

Is this my oil pump or coolant leaking into the oil? Also what do I look for with the coolant leak, now that the heads/gasket have been fixed?

Thanks for any suggestions.

 

You need to find out what the oil pressure really is, and not just depending on the vehicle guage. It could, however be correct, but you really need to know if it is accurate. Coolant leak you will have to locate in order to fix.

 

Welcome to the forum.

Did the shop machine your intake manifold also to ensure that it would mate properly w/ the heads after they were machined?

 

Don't know if they did. Honestly thought it was part of the process. I guess I'll ask to see if they did. Thanks for the help.

 

X2 they should have milled them, wouldn't be much, but you need a machine mated surface for the gasket to seal. Saying that, is the coolent leak from the intake?

 

Don't know the exact process the heads went through while being 'machined.' Regarding the coolant leak, I've never seen evidence of the leak from ANY area outside of the engine. Also, system has always held pressure and I've never had any white smoke from the tailpipe. Only unusual comment recently from my cousin, who changes the oil for me is that during the last change, he said that the oil looked watery, like it lost it's viscosity. If coolant is leaking into the oil, wouldn't this cause watery oil and oil pressure issues while driving?

 

Since water is a non-compressible fluid like oil, then there wouldn't be any noticeable pressure change unless the oil pump started to wear excessively.

If the intake wasn't machined properly, then there's a good chance that there's a mismatch at the coolant crossover and that's where the leak would get into the oil and it wouldn't show from the outside. Without machining, the intake won't sit down as far on the heads and then the ports don't line up and therein lies the problem.

If coolant has got into the oil, then you are probably looking at new bearings at a minimum.

 

coolant in the oil will give the oil a milky color, thin oil is usually caused by too much un-burnt fuel....