I think there are two reasons why I’m in this weird situation. The first is that I communicated to Tracy, the nice lady who bought the Jeep before it unceremoniously blew up on her, that I could have this thing fixed fairly quickly. Yes, the water pump had failed, the engine had overheated, and the cylinder head had cracked, but I’ve replaced so many 4.0-liter Jeep cylinder heads over the years; I could take care of this relatively quickly. The issue is that I run a car website during the day, travel to car-events far more often than I’d have thought (after all, we’re brand new. I wasn’t expecting automakers to invite us to things this soon), and I have my own junkers I have to mend. So I just haven’t gotten around to fixing the Jeep, which left Tracy stranded nearly two months ago. That’s on me for not managing expectations. The second thing is (and I think this is something mechanics probably struggle with a lot): I don’t quite think Tracy quite understands the complexity of this repair. I had to yank a freaking cylinder head off a motor! Thats half the damn motor. Plus I had to send that head to a machine shop, which took four days to inspect the part to find out that it was cracked. Then I had to remove a cylinder head off a spare motor I had laying around, then send that into the machine shop to be checked and milled (to get it perfectly flat, just in case). This is not trivial stuff, but I think she’s used to just dropping her car off at the shop, and them having it done in a few days. And admittedly, a shop that took two months to do a repair would not be tolerated. So I think it’s on me for poorly managing expectations, which I think were quite high in the first place. Anyway, Tracy is great, and I don’t plan to let her down. So the next 48 hours are going to suck.
A post shared by David Tracy (@davidntracy) Here’s where I’m at: My cylinder head has been machined and mostly cleaned. I say “mostly,” because the valves are still in the head, so those intake and exhaust ports didn’t get cleaned all the way. No matter, because I’m removing all the valves, since I have to make sure the valve seats are in good enough shape to seal, and I want to replace the valve stem seals. Here, let me show you what I’m talking about.
The cylinder head that I’m dealing with sits on top of the engine block, which features a bunch of cylinders with reciprocating pistons inside (see above). Air and fuel enters the cylinders via the intake valve (the big one in the image below) and exist through the exhaust valve (the smaller one).
The two valves slide into a hole in the cast-iron cylinder head called the “valve guide.” Here, you can see the two guides clearly in this image:
Where the face of each valve seals against the head is called the valve seat. It’s critical that both the valve’s face and its mating surface, the valve seat, are both nice and smooth — devoid of rust or other pitting. If the two mating surfaces don’t seal well against one another, there will be a leak, which could result in poor engine performance.
So what I’m doing now is called valve “lapping” or valve grinding — basically, I just put a bit of valve grinding compound against the face of the valve, then I push the valve against the valve seat on the cylinder head, and then I spin the valve really quickly. In reality, I’m lazy, so I’m actually just pulling the valve against the seat, since I wanted to use a drill to make things quicker (the rubber hose is just an adapter): This causes abrasion between the valve and head, creating a nice, smooth, silver surface. Here’s the “before” picture:
And here it is after:
As for the valve stem seal, that’s a little rubber cup that the valve stem slides into. Here’s a look at a new one and an old one — the seals are inside the valve springs, which close the valves when the camshaft isn’t actively pushing them down (via, in this case, pushrods and rocker arms).
The seals tend to become brittle with age and heat exposure, allowing oil to run down the valves, through the valve guides in the cylinder head, and into the cylinders, resulting in smoke during startup. I figured I’d replace these just to be safe, since the head was already off.
Anyway, I have to finish off the remaining couple of cylinders, but once the guides are smooth and the seals are in, I’ll pop the 80 pound-ish head onto the engine (that’s gonna be rough), and then I’ll start putting everything back together. First I’ll install the pushrods and rocker arms, which I have organized so that I know which came from which hole (the theory being that two mating parts tend to wear against one another, and you don’t want to reintroduce a part to a different mating partner, or it’ll have to re-wear to its new mate).
Once the pushrods and rocker arms are all torqued to spec, I’ll the cylinder head bolts in and torque those to spec. Then I’ll install the valve cover, snug that down, and get on to the exhaust and intake. I’ll bolt those on, following the right torque spec/sequence, then I’ll install a new water pump and thermostat, then I have to put the power steering pump back in, the auxiliary radiator fan back in, the airbox back in, and on and on. Plus, Tracy wants me to swap the shocks, and anyone who knows Jeep Cherokee XJs understand that the probability of me breaking at least one upper shock bolt is 100 percent. There’s a ton of work ahead, and I have two days to do it. It’s an all-too-familiar situation.
