I think I know the answer to this but....

I've got exhaust gas in the coolant of my '97 4.0, according to my test kit. It was conclusive - the fluid turned yellow pretty dramatically. No apparent water in the oil, but that's no guarantee.

The history: On my way to work on Thursday, and it started running a bit warm. One tick mark above 210, which is where it normally sits. Odd, I thought, so I started keeping a very close eye on it. (I check my gauges frequently, anyway.)

Then it took off like a rocket, headed for 260, and I slapped it into neutral and killed the engine, headed over to the shoulder. I had a bit of coolant splashing down by the right front tire, and a bit had sprayed up on the underside of the hood, right above the thermostat housing. Not much either place. Called for a tow truck. I love unlimited Allstate towing service!

Did a flush today (was going to anyway, put the Amway in it a month or two ago), filled with water and ran it for a half hour with no problem, with no thermostat in it. It ran just a tad above the 210. Did the exhaust gas test, and shut it down.

Other than pulling the head, is there a way to tell if the block, head, or head gasket failed?

 

sounds high huh?, no stat should be cool.

 

Yes, I know. With exhaust gasses getting into the coolant, it will run high.

 

The first test I would recommend performing is a basic compression test. It can often diagnose a bad head gasket.

Easy to perform, you can rent the gauge at a parts store if you don't have access to one.

The compression spec. for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

Report back what you find for comment.

 

How to tell which failed? Block, head, or head gasket?

Ain't happening, time to start pokin.

 

Oh. Duh. I knew that.


I got 145 on all 6 last time I checked. Handy to have that refernece point!

I'll drag out the compression tester and take a lookee.

I might get lucky and hear it hissing out somewhere.....


But yes, Turbo, I am expecting to be pulling it apart to get a final diagnosis. I'm just looking for some advance info. For example, if I do a compression test and find 0 psi on one cylinder, I pretty much know where to look when I pull the head.

Well, I was planning to work from home the next two weeks, so I guess there won't be a problem with one vehicle in the family for a while....

 

While the plugs are out anywho borrow a cooling system pressure tester.

 

a leakdown test will tell you exactly what is going on

 

Compression test:
Cylinders 1-5 are at 145, Cylinder 6 is at 140

Not too bad for 277k miles. Initially, I had been on the road for almost an hour when it overheated, and in testing it at home, it would run for a while before the temp gauge headed for the stratosphere, so it may not show up when cold.


I've got dropping temps going down to 25 tonight and lower tomorrow night, and there is nothing but water in there right now, so I'm going to drain it and forego some other tests right now. In the end, leakdown and compression tests might tell me which cylinder to look at, but it could still be head, gasket, or block on any one of them, so I still have to yank the head to see what's going on.

Unless.... I am going to try pressure testing the cooling system with air (maybe tomorrow) to see if I can find the leak. The last time I had it fired up, when I shut down I tried to listen carefully around the engine, and I thought I heard a hissing on the manifold side of things, but a neighbor rattling an empty trailer down our gravel road obscured it before I could pinpoint anything, and then it was gone.

Oh, well. I have a very minor exhaust leak right at the extreme front of the manifold, so I needed to pull this stuff apart anyway....

If it's the head, any recommendations on new? I have my 94 that I'm parting out. I may try swapping in the engine from that. The engine was fine when it was parked a couple of years ago.

 

I'd say she had a good run for being at 277k with that kind of compression. I'd also say it's most likely the head gasket but if the head is cracked, probably just go with a clearwater like everyone seems to do. Or swap your 94 head on and save a few bucks!

 

Mark go to AutoZone or most part stores and rent/free when returned a leak down tester, pull the radiator cap put it on and pump up and leave it over night if needed or not so long if real bad.


Good luck!

 

With all do respect sir, he would be looking for a cooling system pressure tester to do this job, not a leak down tester.

 

leak down test is done at the cylinder. there is an acceptable tolerance on a healthy engine, but 99/100 it's not being done on a healthy engine and the issue is obvious. you can hear air leaking in different areas which will then tell you what is not sealing.

 

Pressure test the cooling system with the proper tool.

System has to be full of liquid though.

 

Head's off. I don't see any obvious problems with the head, gasket, or block. I have not had time to inspect the walls of each cylinder. I will do that tomorrow.

Still, I expected to see some sign of exhaust gasses moving from a cylinder to a coolant passage. A crack in a casting, or some marks on the gasket. Nothing so far.

???

It takes a while, though, so it's probably heat related. Perhaps a slightly warped head? But then I'd still expect to see some marking on the gasket....