Hi there,

I just bought a 91 F150 XLT Lariat, Auto, 2WD...either 104K miles or 204K miles not sure which. This is my first truck, and I'm kicking myself for not being smarter about the purchase process. I should have brought someone with me who knows more about auto mechanics....as it was I went and checked it out....while it was a little rough around the edges...nothing too bad. It runs and drives, but at the time I didnt realize how rough it was running. There is a ticking/knocking from the engine. Anyway $650 later I was the proud owner. Got it home.....dropped it off at a shop to have them take a look for me. Just heard back from them.....confirmed with oil pressure gauge there is no to low oil pressure in the engine at idle.

What the shop is telling me is that the engine is likely shot, and with what it would cost to rebuild I'd be better off putting a new engine in, a swap maybe?......they are saying it's toast and while I can limp it along for a bit it's a ticking time bomb basically. I asked them why can't we try just putting in a new oil pump and see if that doesnt help....they told me it's $1200 to replace the oil pump and the pump is inside the engine?

 

Did the mechanic say what was making the ticking/knocking noise? What is the oil pressure when revved? If it is a rod bearing, the engine is toast for sure. You might try putting some 20w50 oil in it and see what that does. What engine is it anyway?

 

 

Which engine does it have?

 

 

If it is a rod bearing, you will find out soon. I would still contact the mechanic and ask what he thought the sound was caused by. You need to decide if you want to dump a lot of money into repairing the truck or pass it on to someone else before dumping the money.

 

A new oil pump isn't going to fix anything but make the wallet lighter. Because you know nothing about the engine other than it didn't exactly run great, it can easily be faulty injector(s) that has dumped a significant amount of fuel into the oil. The oil may be thinned out due to fuel dilution. I'd have the guy change the oil using Rotella 15w-40 oil and a new filter. That may restore the oil pressure at idle. Because I own a 91 Bronco since new and it's also a 5.0/302, these engines were never quiet. They have always had a bunch of rattling around in the engine even when new with zero miles on the speedo. Mine has had the "rod knock" for the last 135,000 miles and compression is still at factory. If you want to have your mechanic pull a compression test on it, that will give you the real story about the engines worth. You won't run one but for a few miles with no oil pressure. The rings will be toast meaning low to no compression. I'd be there when he does the compression test if you decide to have him do it. If the bearing are worn enough to knock, the rings are long gone.

 

If the compression is good, the rough running could just be a need for a full tuneup - plugs, wires, cap and rotor, possibly dirty injectors. The oil pressure does need to be addressed first.

 

Thank you, and I do want to do a compression test also. But I guess that won't tell me if the main bearing or thrust bearing is the source of a problem. If I do the oil pump (the part is pretty cheap and I'm going to attempt to do it myself at a self service garage) and that improves oil pressure ive made some progress right?

 

If you insist on changing the oil pump, use a high volume oil pump. It pumps more oil and the idle oil pressure will be higher.

 

x2 on Roadie's comment...

A lot of people swear by Melling pumps... you'll need this one: Melling High-Volume Oil Pump M68HV

 

 

IF you decide to change the oil pump yourself, there's a few things you need to know. It's not simple unbolt the pan and swap the pump kinda fix. You change it two ways. One, remove the intake manifold from the top of the engine. The reason: you have to jack up the engine about a 12" to get the pan out. The manifold will hit the cowl long before the engine gets high enough to remove the pan. Then you have free range to get to anything on the bottom of the engine. Two, leaving the tires on the ground, jack up the body as far as you can. Then unbolt the pan. It won't come off but you can reach in and unbolt/swap out the oil pump. A novice with reasonable mechanical abilities should be able to change out the oil pump in 4 hours. The $1200.00 is stupid expensive. I'd find another shop as these clowns are clueless.
In regards to the high volume oil pump, I wouldn't. Use a standard pump. You may incur other issues with a high volume oil pump restoring full oil pressure. Leaks will be your biggest issue at tired gaskets and seals. If you rebuild the engine, then a high volume pump is great insurance for a long life engine. I run a modified oil pump in mine. I idle at 70lbs and run at 70lbs, hot or cold. But I had the pleasure of being able to do it on a flow stand in a lab before it got put on the engine. You'll also want to avoid the high pressure pumps in any scenario with a 302.
FWIW, these old 302s will run forever with hot idle oil pressure at at 10lbs. That's at idle which is 550 rpms. As rpms drop, so does the oil pressure. Might want to check the engine idle speed before starting any fixes.

 

Labnerd has valid point.

I know it's not a 302, but for example, my Harley service manual points out (clearly points out) that there should be sufficient (rated idle) engine RPM for the pump to work properly OR damage can occur to the engine.

So for the folks that like to adjust the idle speed to lope/potato-potato their Evo engines in their Harleys, they are doing nothing but damage because the oil pressure is so low.

Same concept.