Need expert help with engine diagnoses

Fryguy302 · December 3, 2006

Summary of my questions:

1. What can cause HC to show up in the coolant other than a head gasket?

2. Can a head gasket be blown or a head cracked even when the truck runs great, has great power, doesn't use coolant, and passes cylinder pressure and coolant pressure tests?

I'd much appreciate it if any experts can answer the above two questions for me. If you want to read the long version, here it is:

Trying to figure out my newly purchased 96 Yukon. I know the previous owners of this truck and know the history on it. For the past year, it's been having a problem starting when cold. It is NOT the typical fuel pump or regulator or ACT sensor issue, because the problem is the way in which the truck CRANKS when cold. When it's cold, it cranks fast for maybe one revolution, then cranks really slow (too slow to start sometimes), almost like something is binding up or the starter is failing. The starter has been changed twice since this problem presented itself about one year ago. The battery has also been changed. The previous owners continued to drive the truck every day like this.

When I bought the truck, I dropped it off at a local shop for diagnosis, expecting them to find a bad ground or a bad positive cable, or maybe a bad starter.

They shocked me by telling me that the head gaskets were probably blown or a head was cracked. They told me they put a wand to the coolant and there were hydrocarbons in it. They also told me they did a propane test and there was an intake leak. When I asked how that affected the starting, the tech told me that he thought maybe it was actually hydrolocking because of coolant in a cylinder. He told me I should probably just put a new motor in the truck, since it has 100k on it anyhow.

At this point, let me tell you how great this thing runs. The oil has been changed every 3k miles for the last 65k (that's how long the previous owners had it). It idles quietly, runs and drives great and is very strong. When I picked it up from the shop, I had my buddy drive it home, and I followed him, so that I could look for smoke. He stood on it one time, and not only was there no smoke, but my brand new 5.4L F150 could not even keep up with the old Yukon, he just walked away from me, even though I had my new truck floored.

We got it home and did the following:

1. Pulled a valve cover just to look. No milkshake and not even any sludge. Looked like new. Oil on stick looked great also.

2. Pulled plugs. No sign of any cyl. burning coolant.

3. Did a cylinder pressure test. Lowest hole = 190, highest hole = 195. I've put seen brand new motors which were less consistent than this.

4. Did a cooling system pressure test. Dropped 2psi in 9 minutes.

The previous owners were shocked, and swear that this truck has never overheated or used coolant. I drag race as a hobby, and run a Mustang that's had many different superchargers on it and is now turbo charged. I've blown a LOT of head gaskets in my day, and I've never seen one like this with no symptoms.

Please let me know what you guys think. Any help would be greatly appreciated.

GM-Tech · December 3, 2006

Nothing other than a head or head gasket will cause hydrocarbons in the cooling system. The coolant doesn't produce these gases, so where else would it come from. And yes, they can run great even with a tiny leak of coolant into a cylinder.

The tech you're talking to sounds fairly knowledgeable. I was going to say the same thing about hydro-locking on the first start. Overnight, coolant could seep into a cylinder and build up enough to cause this.

Here's what I'd do. Get the engine to operating temperature/pressure and leave it sit all night with ALL the spark plugs out of it. Get two friends, one on each side with a GOOD light, to watch the plug holes and crank it over the next morning. If there's a leak into a cylinder, someone will see a mist (or more!) come out of one of the plug holes during the first couple revolutions of cranking. Don't try to watch this all by yourself because you may miss it if it's on the opposite side.

It's a little unusual that you're not seeing anything in the valve covers. Not even condensation? I would say that a very small leak is probably all being consumed by the combustion process so it's not enough to contaminate the oil.

There's something I swear by called "Block Check". Use this only if you're unsure about their test for HC using the gas analyzer. They used to sell this at NAPA, but you may be able to find it at any good auto parts store. It's a blue liquid that you put into a device that looks something like a turkey baster with an old carburetor style sintered bronze gas filter in the bottom.

Get the engine warmed up and draw some air from the radiator (you may have to lower the level first since you do not want to contaminate the test fluid with coolant) and this turkey baster bubbles the air up through the liquid. Depending on the amount of HC, the liquid will turn from a rich, royal blue to green. The more HC there is, the lighter green it will get. If it's really bad it'll get to an almost clear yellow. Sometimes the change to green is very slight so you might have to compare new, uncontaminated fluid with the test results fluid to see the difference.

Block Check has never lied to me and I use it on every overheat job just to make sure there was no head/gasket damage. It used to be $35-$40 or so for the complete kit, but it's been a very long time since I bought the whole thing. I only need the liquid refills.

The only part that gets me is that you say it uses no coolant. A small leak may not use much, but it should eventually go down. You may have to mark the level to be sure and check it cold in the morning before starting and do this over a significant period of time. One or two days may not do it.