Engine Woes

[cueball]

Well, I just bought a 90 GW. It is a nice truck and I thought I got a really good deal on it.... Until I got it out on the interstate and the engine started knocking!! :eek: :eek:

Need some help with diagnosis. When I start the truck, I notice a short knock which immediately goes away. It is not present at idle or until approx 3000 RPM. At 3k it develops a distinct knock. If I accelerate, it goes away, if I let off the gas, it goes away. It seems to only be present at steady speed.
:confused: :confused:

Has anyone run across this?

I sense that it may be the oil pump. The gauge is erratic & jumps all over the place. My buddy thinks that the pump may have wear and is not holding pressure. I was talking to the counter guy at Autozone yesterday and he thinks it is the crank thrust bearing. He was the gloom an doom type though, you know, "Oh, its an AMC 360, you'd better just junk it now."

By the way, the engine runs strong... really strong. WTH??

[ December 13, 2003, 08:22 AM: Message edited by: cueball ]

[Bob Barry]

How many miles?

If it was main or rod bearings, acceleration would make the knock worse, not better.

Feel free to slap the counter-monkey at Autozone silly. It could be the crank bearing; in which case, you either rebuild the motor or drop in a crank-kit.

The cylinder-walls will usually maintain good seal to the rings long after the bearings are gone. That is why performance won't necessarily drop off.

If it is spark-knock, then it may be as simple as an EGR valve stuck closed, or clogged EGR passages in the intake, or a vacuum leak.

How many miles on the motor?

[carrotman]

With a 90 GW at 3000 RPM, that must be about 100 MPH. It's been a while since I had my 76 that fast, but if mine only made noise at that speed I would be happy. If your's doesn't knock on acceleration, I would rule out spark knock. Same with main bearing or connecting rod knock. I think your oil pressure is electric so check the wiring to it. Also check the oil level.

[cueball]

The truck has 127K miles.

I am estimating the 3000 RPM, I don't have a tach. I starts about 50 mph in third gear. What is that about 2500? I am guessing it has 2.73 gears in it. Don't most of them?

It definately does not do it on acceleration. I have ruled out spark knock. I agree that the oil pressure gauge may be a problem. My next step is to hook up a manual gauge and see what I have got.

I am somewhat adept at diagnosing motors although I have never torn one all the way down before.

Has anyone messed with the oil pumps in these? Does it make sense that it could be the culprit?
If it is not pumping well to the upper half, couldn't that cause the symptoms I am getting?

[joe]

Just a wild guess here but I'd bet on oil starvation at speed and what you're hearing is top end rattle. The cam and cam bearings in these AMC motors are made of melted down old carbs I think. Worn cam and or main bearings will cause low oil pressure.

[Rogue]

problem number 1 - "I was talking to the counter guy at Autozone"

problem number 2 - "The gauge is erratic & jumps all over the place" - while this can be caused by a malfunctioning sending unit, nicked or worn out cam or main bearings will cause this also, I recommend hooking up a mechanical gauge - or what joe said

problem number 3 - having to pull the engine for cam bearings - this just sucks but...you still need to do it if you want to save this engine

good luck

[cueball]

Ok. Gonna hook up a manual oil pressure gauge and see what kind of pressure I am getting at idle, and at speed. Once I see what I get, I will post the results.

I would appreciate it if all of you could weigh in again with your insight.

If I have to pull it and go through it, I will but I don't want to if I don't have to.

By the way, Rogue Star, I have a 69 Ghia Convertible too.

[Rogue]

man first time i saw a ghia i thought it was the ugliest Great Googley MoogleyGreat Googley MoogleyGreat Googley Moogley POS i ever saw

than I drove one

now I own one!!!

[SBJeep]

Not a guru here. BUT, if it were the thrust bearing or low oil pressure, why would the knock go away when you accelerate or let off, and only do it at a steady rate? Does it do if you floor it and run it up through the gears?

You also said you hadn't ruled out spark knock. At a steady rate, vacuum advance could cause this. I've tinkered with mine many times and it there's too much vac advance, it will ping heavy on the freeway at a steady rate, then go away when I accelerate. This sounds different than my lazy lifter that knocks in the morning when I start it up.

My gauge freaks out sometimes, too. It'll spaz and jump up over 80 and back down. It does this totally randomly.

[carrotman]

Could be the vacuum advance. Someone said that the 90 GW had full manifold advance at cold idle to smooth out the idle, then switched to ported idle when warm. What if the dizzy is seeing full manifold vacuum advance when warm? That could cause pinging at steady cruise, but disappear when accelerating.

[cueball]

Rogue Star - I wasn't a VW fan either till I got my ghia. They are impressively simple vehicles.

On the GW. This insight is very helpful. Hopefully I will get some time to mess with the truck tomorrow. I have only had it for 1 week and traveled all week. I don't want to jump the gun and tear into the engine until I have thought about it good and long. This isn't my daily driver so I have time.

Thanks for all the input guys.