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Buying A Used Engine For Your Jeep

Posted in How To: Engine on May 2, 2014
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Questionable yard art or a second-rate jungle gym -- that’s basically what a Jeep is if the heart of the beast, the engine, won’t work. The fact is that engines wear out over time with use or abuse. That’s too bad, but it’s the reality of playing with Jeeps or almost any machine. The good news is that there are more engines out there that might be what your ailing Jeep needs. The problem is that without trusting the seller of a used part such as an engine, it can be tough to know if what you are buying is good or junk. Believe it or not, those of us here at Jp have bought a used engine or two and are happy to help you in your quest for a new mill to power your heep out of the yard. Follow along as we talk about things to look for and things that should set off warning bells when buying a new-to-you used engine.

When buying a used engine, the best way to make sure you’re getting what you need is to hear the engine run before it gets pulled from the donor vehicle. Listen for ticks and knocks that could indicate that the engine needs a rebuild, a new timing chain, or is just junk. Small ticks are probably not a big deal unless you plan on running the engine as is. Large knocking noises can be bad. If the engine turns over but won’t run, that might not be a deal breaker, but it could be hiding low compression or internal knocks that you wouldn’t know about without hearing the engine run. Look at the oil to check if it’s nice and honey- colored or black and worn—also give it a smell. If you can smell fuel in the oil or it looks foamy and whitish, that could indicate coolant coming from a bad head gasket. Any dubious evidence should bring the price down or help you decide to keep looking.

If all you can track down is an engine that has already been pulled and you can’t hear it run, all is not lost. Put a big wrench or socket on the front of the crankshaft and try to turn the engine over a few times. If the engine has good compression and the spark plugs are installed, this is gonna be difficult. If there are no plugs in the heads and the engine won’t spin, the rings could be rusted to the cylinders or there could be internal engine damage that is not allowing the engine to turn over. If the engine will turn over and everything feels tight, the engine may run, or at very least, be rebuildable. Worst case scenario is that if the engine has been rebuilt a few times or is really worn out, it could turn over and still be junk or prohibitively expensive to rebuild.

If you know what you are looking for, you can also “read” the spark plugs from the engine if the seller is alright with you pulling the plugs. Just make sure to keep track of which plug came from what cylinder, so if there is an issue you can follow up with further investigation such as a compression test. Oil on a plug is a bad sign, as is a fair amount of white chalky residue on the spark plug’s electrode. Both may indicate worn rings, valve train issues, or a bad head gasket.

If you have decided to take the engine home with you because you think it will run or is a good rebuild candidate, there are some other things you can do once you get home. With the crank case full of oil, wire up a battery and spin the engine over a few times. You can now run a compression test, but the numbers could read low (as you are supposed run a compression test at operating temperature and your new engine is nowhere near operating temp). If you have any low cylinders, squirt some oil through the plug holes. If the compression reading goes up, suspect a leak between the cylinder wall and rings. If it doesn’t, it may be a leaky valve or blown head gasket that’s allowing the leak.

If an engine has been sitting outside in the rain without being sealed off from the environment, chances are that water has gotten in to it and the whole thing could be rusted solid (rings rusted to the cylinder walls and the crank rusted to the block). If that’s the case, it’s probably only good for scrap metal. If you are stubborn and want to try to save the engine, you can try soaking the cylinders in penetrating oil for a few weeks and then seeing if it will come apart. Chances are you will be wasting your time—unless you are in the scrap metal business.

See the small round gizmo in the freeze plug of this AMC 304? That’s a temperature sensitive indicator that was installed when this engine was rebuilt at some point in the past. The indicator was installed to tell the commercial engine rebuilder if the engine was overheated after installation. It also tells you, the used engine buyer, that the engine has been rebuilt at least once already. That might mean that the engine may have overbored cylinders or under-sized main bearing journals and may not be rebuildable. Or, if the indicator looks good and new, that the engine has recently been rebuilt.

If compression numbers are really low, you may want to rebuild the engine. If compression numbers are good to alright, you’ll have to run the engine to see if it has truly bad compression or knocks. You could run a cylinder compression leak down test as well to further help any engine rebuild decision-making. Beyond that, pulling the heads and having a look inside the engine is the next step in deciding if the engine is in need of a rebuild; look for obvious signs of wear and measure cylinder bore tolerances.

If an engine block cracks, it’s generally junk. This can be the result of spectacular engine failure or can be caused by freezing weather or improper coolant in the engine’s water jacket. This L-head cracked just below the distributor in a couple of places because the coolant froze and expanded. Then someone tried to weld up the cracks. It didn’t work, and this engine is only good for scrap metal.

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