Tontar

I have a 2002 Grand Cherokee, 4.7 H/O engine. A week ago, I got the dreaded engine light symbol, and immediately hooked up my OBD 2 meter and it said "Ignition Coil 8 Circuit". I pulled into an O'Reilly's, bought a new coil and installed it. Can't believe I did that in the parking lot. Cleared the memory, started up, and it ran fine. Until, that is, I pulled onto te road an accelerated. Same thing, same result. The engine ran rough, a missing cylinder, and the same #8 ignition coil 8 circuit warning. Pulled into the next O'Reilly's a couple miles down the road. Bought a new sparkplug, did the swap again, and it ran sweet again, until I pulled onto the road and loaded the engine. I turned around and drove home, no play day after all.

Once home, I popped the hood, and looked at the engine. The coolant reservoir was bubbling lightly. Some sort of air coming from the radiator. I popped the lid to the reservoir and sniffed it. Gas fumes. It smelled like fresh mixture, not exhaust. Gas fumes. Recently, the engine has been consuming coolant, and dramatically so most recently.

So the symptoms I see are:
1. OBD alerts saying "Ignition Coil 8 Circuit" and "Cylinder #8 Misfire".
2. Coolant being consumed with no apparent leaks.
3. Gas fumes in cooling system.
4. Idles fine and smooth, after sitting and runs rough when loading engine with same alerts being triggered.

I'm not anywhere near an expert at this stuff, which is why I'm here asking, but my guess would be that I have a blown head gasket that is leaking between the #8 cylinder and a cooling passage. That would explain the back and forth exchange of fuel mixture and coolant, I think. That on the intake stroke it's sucking not only mixture, but coolant. And on the compression stroke it is pushing mixture into the coolant passage. On second thought, maybe it's not sucking coolant during the intake stroke, since the intake valve would be open and the least resistance would be fuel mixture. So maybe it's sucking fuel mixture just fine, but then during the compression stroke it is forcing mixture into the coolant system, and once that happens, pressure is relieved in the cylinder, so when it does the expected power stroke, having no compressed fuel mixture, and having the valves closed, it actually creates a suction stroke pulling coolant into the cylinder during that phase. That way, I get fuel mixture forced into the coolant system, and coolant being sucked into the cylinder during the power stroke. When the valves are open for intake and exhaust, those ports are big enough to be the route of least resistance for filling and evacuating the cylinder contents. That's why I get no exhaust fumes in the coolant, and no coolant during the intake stroke. Only coolant during the power stroke, which is then kind of forced out during the exhaust stroke. Also, if the head gasket is compromised only between a cylinder and coolant passage, i'd likely not see coolant in the oil nor oil in the coolant. Sound reasonable? Or am I totally an idiot?

I am pondering doing a compression check, but OMG, it would have to be the #8 cylinder that is showing as problematic. It's hard enough to get a coil or spark plug in that one, with the dipsticks in the way and it being such a tight, closed area. There's clearly a problem, with a lot of coolant being consumed and and the engine running on 7 cylinders. Am I a fool for assuming, based on the symptoms that it is either a head gasket, cracked head, or cracked cylinder without doing a compression check? That either way, whether it is A, B, or C, I'm going to have to pull the head(s)?

And finally, given that a diagnosis can be arrived at, any helpful hints before I start tearing this thing down? Thanks in advance for any helpful advice. Aaron

boxburn

You would be better posting in the Grand Cherokee forum here.

However, I would say what you have concluded sounds very likely and a compression test would be a good start. If you find low compression as you expect in #8 then a leak down test would be good to do too. With that if you are leaking into the coolant passages you will see it bubbling through during the test.

Tontar

Thanks! Will do.

martinbuilt

Also, check the wiring of both the coil and fuel injector close to the connectors. Sometimes the insulation comes off and the wires short together.

Tontar

Thanks, Martin, I'll do that as well. I will be checking everything I possibly can as I dig into it. I tried to see if I could find anything wrong with the wiring when I swapped teh coil, and then again when I swapped the spark plug, thinking what else might be causing a misfire. I was worried that might be the problem, but I couldn't see anything wrong with the wiring, at least as far as I could tell at the time. And of course, that wouldn't account for the coolant consumption nor the fuel mix in the coolant.

martinbuilt

I must have not read the part about the coolant and the fuel mix. That does sound more like a head gasket.
Martin

Tontar

Thanks, Martin!