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Jeep 2.5L I-4 Engine Plastic Valve Cover Fix

Posted in How To: Engine on January 3, 2017
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Photographers: Trenton McGee

It’s well known that, years ago, some genius at AMC decided it would be a good idea to put plastic valve covers on Jeep 4.2L (258ci) engines. Unfortunately, it didn’t take long for them to start leaking. What’s less known is that the genius decided to do the same thing with the AMC 2.5L four-cylinder. While the aftermarket long ago addressed the needs of inline-sixes with nice aluminum valve covers to replace the leaky plastic, the four-banger owners were left out in the cold with perpetual puddles in the driveway. While quite a few of those 2.5Ls were long ago replaced with small-block Chevys, AMC V-8s, and other popular engine conversions, there are still quite a few four-popper–powered CJs, Cherokees, Comanches, and early YJ Wranglers running around and marking their territory wherever they go. Luckily, there’s a cheap and easy junkyard fix.

We already knew that an engine swap was coming sometime in the near future when we acquired our new-to-us ’84 Jeep CJ-7 with a 2.5L, but having the less desirable engine led to a more desirable price tag. We wanted to enjoy the Jeep while we were gathering parts for the engine swap, but the oil slick it left in every parking space was both annoying and expensive due to constantly topping it off. One day, a friend wondered aloud if the aluminum valve cover from a later 2.5L would bolt in and replace the plastic one, so we decided to find out. For $10—or less than the cost of the two quarts of oil we were dumping into the engine every other week—we acquired a junkyard-fresh aluminum cover, and 45 minutes later, we had an engine that leaked a whole lot less. The total cost was just under $35 only because we went all-out with a new gasket, bolts, fill cap, breather, and a bit of breather hose. Had we stuck with just a gasket and the valve cover, the whole shebang would have been less than $20. The swap didn’t stop all of the engine oil leaks, but it did fix all of the leaks coming from the top of the engine and reduced overall leakage by more than half. Keep in mind this swap works with only the AMC 2.5L and not the earlier GM 2.5L Iron Duke engines, which have tin valve covers anyway. Here’s how we did it.

Plastic valve covers are just a bad idea. They’re known to crack, but they also deform due to engine heat cycles and from people torqueing the mounting bolts too much. Though ours wasn’t cracked, you can see there was a steady stream of oil leaking out from under the valve cover and on to the rest of the engine. The lack of power is hard enough to deal with, but the constant puddles of oil only added insult to injury. Fortunately, later four-cylinder valve covers swap directly in place of the offending plastic units.
As near as we can tell, plastic valve covers were used on ’84-’90 carbureted and Renix-injected AMC 2.5L engines in CJs, MJs, XJs, and early YJs. The switch to an aluminum valve cover happened at the same time as multi-port injection, which was for the ’91 model year. We used an aluminum valve cover from a ’91 Wrangler, but any valve cover from a ’91-’02 XJ, YJ, TJ, or Dodge Dakota with a 2.5L engine should work.
Before diving into the swap, we spent some time degreasing the engine so we would be working with something less nasty. Once done, we removed the valve cover and thoroughly cleaned the gasket surface. Be sure you use a gasket from a later (’91 and newer) application. It’s a much better gasket, and it’s designed to be used with the aluminum valve cover.
If you think about it, grab the valve cover bolts from the donor. The ones with the plastic valve cover are too long or too short; we don’t remember which. Failing that, they are 1-1/4-inch 1/4-20 bolts that can be picked up at a hardware store.
The crankcase breather configuration on your engine will probably be a little different than what is compatible with the aluminum valve cover. We took the opportunity to address the cobbled together breather that was on our engine and replaced it with a normal hose connecting the new valve cover breather to the air cleaner.
The later valve cover looks right at home on the earlier engine. Note that if your engine had a PCV valve, that hose needs to be connected to a specially ported nipple that will be present on the later valve cover. We managed to lose the nipple when we were cleaning up the valve cover, so we temporarily installed a plug until our next trip to the junkyard, at which time we grabbed a replacement.

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