Today I got the results of my first UOA on the 99 Surburban 7.4 we bought last year to tow the GF's horses around ( link ). One year on the oil, 1500 miles.

As you can see from the UOA, there's a coolant leak. I'm hoping this is one of the engines that has intake gasket problems because I really don't want this to be the head gaskets. I may give it a compression check this weekend if I can find the time (although I'm not looking forward to trying to get at the rear spark plugs) to verify.

It was visibly loosing coolant for a little while, but it was after I'd drained and replaced most of the coolant when I replaced the thermostat, so I figured it was just burping all the air out of the system. The coolant level has not fluctuated for several months now (probably the last 100-200 miles). So, it's not putting a huge amount into the oil at this point.

I had a few questions:

1) If it is the head gaskets, will a compression check definitely show it?

2) If it's not the head gaskets, is it likely the intake gasket or could it be something else? What can I really do to figure out what the problem is?

3) How involved is the intake gasket replacement? Is it a DIY'er friendly task or am I better off taking it to a mechanic?

4) Do I need to treat this as a "don't drive the car except to the mechanic to get it fixed" problem? The GF was planning on towing her horses to a show next weekend. It would entail about 100 miles of driving, mostly towing a two horse trailer, including some high load driving up a grade. I'm not too thrilled about that prospect but OTOH I'm not sure an extra 100 miles are going to hurt anything. Who knows how long this has been going on, anyway.

oil in coolant or coolant in oil?

I would not drive it very far. get it fixed

I would get it fixed first. The coolant leak could be alright, then while your girlfriend is towing horses up a hill whatever of a seal is left decides to let go when the engine hits 220...

Wow, total bonehead typo. Coolant in oil. If any mods see this and want to change the topic title I would be grateful :)

This is basically what I'm thinking, but I wanted to see if I'm being overly cautious. I doubt I'll be able to get it fixed before next weekend although I suppose it's possible. Most likely she'll have to bum a ride with somebody else to go to the show.

My guess is intake manifold. Not sure you can do much to really pinpoint it other than replace it. It could be a pinhole size and never visable. But first do a cold & hot compression check and see if you are getting any leaks in the cylinders.

But do not wait...fix it now.

Also,Mark the distribitor before pulling out if you plan on doing the job,the intake gaskets.The distribitor is set with a scan tool,not with a timing light.I am sure the upgraded metal intake gaskets are out for the 454.The upgraded gaskets are out for the vortec 350s and 4.3 including the 3100s and 3400s.Does it miss?If not,not the headgaskets

Most likely it's an intake gasket.

Another possibility is a leaking oil cooler in the radiator but that would show up with oil in the coolant too.

A headgasket or cracked head is a possibility but the old 454's are pretty tough. It takes a lot of overheating to hurt one.

Replacing the intake gaskets isn't a bad job. While you have it off replace the heater hose quick connects because replacing them in the truck is a nightmare because they break off.

Atleast if the manifold is off you can work on it on a bench and not on your knees under the hood.

I'm not positive but I don't think it has an engine oil cooler in the radiator. It has a separate auto transmission fluid cooler and a power steering cooler, IIRC, but I haven't noticed any engine oil lines into the radiator. Could be wrong.



It idles slightly rough but I've never felt a detectable miss or any running problems while moving. Also, I don't think this truck has a distributor -- I thought I remembered noticing a computer controlled coil pack but it's buried at the back of the engine so I could be wrong.

The intake manifold looks fairly covered, so it looks like kind of a PITA. Is this realistically going to be an all-day thing? The most involved things I've done before are radiator replacements and fuel injectors (which on a 280zx is more difficult than it sounds like). I'm not known for working quickly.

I'm not particularly keen on paying someone to replace the gasket and then finding out that wasn't the problem though.

I was just looking through the Chilton's manual at the intake manifold r&r instructions, thinking it looked like a decent fun job. Then I read, "Discharge the AC system" .. gah! Seriously?!

Looks like you get to open every major refillable system except the power steering and the windshield washer! It's becoming less and less likely that I'm going to do this myself. My Primary history is with inline engines and even the compression check on this thing looks like a PITA with the height of the thing and how the plugs are low down, buried between the exhaust runners.

You probably dont have to drain it, really. Its more likely you can unbolt the compressor and move it off to the side. That what I do if I have to Go Deep.

The 454 has a distribitor.ANtonother problem you might encounter is loosening the EGR tube,seized in very good and the EGR tube with a fitting might have to be replaced

I just went out and looked under the hood again.



Yeah, that looks like a possibility in this case. It looks like it's mostly just the flexible hoses that are in the way.



I stand corrected on the distributor. Like I said, it's buried, and looking at it before I saw all the plug wires coming off parallel to each other and assumed it was a computer controlled ignition module like I've seen on some Fords and Subarus. Looking at the manual now I see it's just a distributor with a strangely shaped cap. How archaic.

Thanks for the tip about the EGR tube.

One thing that I can't seem to ignore here is that the coolant has stayed at pretty much the same level in the expansion tank for several months now, and the radiator is still full. The coolant does not look out of the ordinary. I suppose I shouldn't tempt myself to let it be driven though...