1990 Chevy 5.7L V8 cam and valve setup?

[Elkins45]

I have a relative who is still driving a 1990 C1500 Chevy truck with the 5.7L V8 engine. It has throttle body fuel injection and still has conventional distributor ignition. He tells me it’s been running badly so his mechanic replaced the plugs, wires and rotor button which improved performance somewhat but not enough. The mechanic says he thinks it’s a camshaft problem and cylinders 4 and 6 are losing compression. $1500 to repair or $2500 plus labor to replace, and the truck has 180k miles. He doesn’t have a lot of money but I just don’t see putting any money into a 36 year old Chevy nearing 200k miles.

“Camshaft problem” causing loss of compression? I know I’m a complete novice about this stuff but how does a worn cam lose compression? I can see worn valves or stuck lifters, but worn cam? These are pushrod engines, correct?

Thoughts?

 

[Tman220]

These engines have hyd flat tappet lifters/cam. While it's possible it has rounded off cam lobes, worn lifters, those engines weren't known for that. If the heads and cylinders check out with a leak down test, then perhaps rounded off cam lobes are a possibility.

The aftermarket for those engines are fairly common i suppose. If you can find a cam with close to factory specs it may be worth it to fix it if the rest of the truck isn't rusted out. Tbi systems don't tolerate cams with substantial differences in duration, LSA, etc without tuning with software nobody really possesses anymore.

 

[clinebarger]

Nothing a dial indicator can't verify. Those cam lobes go fast when they do go from the lack of lifter spin....But like Tman said TBI truck engines went the distance more often than not. The oil/filter would have a ton of metal.

Just about all of them need a throttle body & distributor rebuild if it hasn't been done.

 

[dfarr67]

I would start with replacing the cap- why just the dizzy button.
If it's the cam- some were roller cam ready blocks which would be easy to change if you are doing the cam and lifters anyway.
If the mileage is up there- EGR, leaks at TBI shaft, spark module- lots of things.

 

[Chris142]

Im going to ASSume a "rotor button" is the rotor inside the distributor cap. If so it's due for the cap too.

If an intake lobe is flat it won't let the intake valve open and compression will be down as it can't suck air and fuel in.

If an exhaust lobe is flat it will have more compression as the air and fuel can't leave.

A flat cam on one of these engines very rare. I recommend a new cap then pull the valve covers and run it.

If it is indeed flat it really needs a complete rebuild to remove all the metal from inside the engine.

 

[dfarr67]

Back in the 80's all kinds of GM 5.0L cams went flat- I don't recall owners rebuilding engines over it. If the truck isn't burning oil why bother with more expense?

 

[crashz]

I've heard of plenty of small block cams going bad and nearly as many people just swapped the cam and lifters. Not something I recommend, but its been done.

Is the truck in good shape? How's the transmission (also nearing 200k)? I think your friend should look at the costs vs replacing in kind. It really might favor repairing it.

 

[Chris142]

Back in the 80's all kinds of GM 5.0L cams went flat- I don't recall owners rebuilding engines over it. If the truck isn't burning oil why bother with more expense?

All that hard metal gets pumped around the engine destroying the soft bearings, ya some people did just replace the worn parts but the proper way is to get all that scrap metal out.

My 350 lost 5 lobes at the same time. That was a mess.

 

[dfarr67]

All that hard metal gets pumped around the engine destroying the soft bearings, ya some people did just replace the worn parts but the proper way is to get all that scrap metal out.

My 350 lost 5 lobes at the same time. That was a mess.

I don't disagree- depends on the vehicle, mileage and $$. If it passes compression and doesn't burn oil- I mean they really are a stone age engine and not fussy.

 

[Elkins45]

I've heard of plenty of small block cams going bad and nearly as many people just swapped the cam and lifters. Not something I recommend, but its been done.

Is the truck in good shape? How's the transmission (also nearing 200k)? I think your friend should look at the costs vs replacing in kind. It really might favor repairing it.

I don't know what shape the transmission is in but I do know the paint is bad. He had rattle canned it in the worst spots the last time I saw it, which was last summer when I replaced the windshield washer lines that had rotted into pieces.

If it were mine I would thank it for it's long service and send it to the scrapyard but I don't know if he has enough available cash to buy something better because he just dumped a ton of money into home repairs.

 

[Arc]

Take the valve cover off and look at the rocker arm movement. If a cam lobe is worn it's noticeable.

 

[AdmiralYoda]

The mechanic needs to do some more basic diagnosing before calling it a cam failure. And a cam failure usually requires an engine rebuild because of all the metal trash circulating through the engine that would easily eat those old bearings.

Even still, a flat lobe or lobes on a camshaft wont cause low compression as the valves will still close just fine...they just wont open as much. A leak-down test can verify if the loss of compression is due to poor ring seal or intake/exhaust valve seal.

As @Arc mentioned the next obvious step is to take off the valve covers and look for loose rocker arms. If a cam lobe went flat they will be very loose, if they are in good shape they should be snug. And if a cam lobe went flat it would probably have a lot of valvetrain noise.

Lastly....I'm pretty sure GM introduced roller lifters in 1987/1988 on 5.7's. So it should be a roller cam, not a flat tappet which drastically lessens the chance of a wiped cam lobe. A collapsed lifter could cause that though. Still...that can be diagnosed by taking the valve covers off.

I'd verify the timing too. Those TBI's were known to be finicky as well and can cause an engine to run rough.

 

[Tman220]

Lastly....I'm pretty sure GM introduced roller lifters in 1987/1988 on 5.7's. So it should be a roller cam, not a flat tappet which drastically lessens the chance of a wiped cam lobe. A collapsed lifter could cause that though. Still...that can be diagnosed by taking the valve covers off.

1987 was the first year for Hyd rollers on the L98 350's used in corvettes and F-bodies but flat tappets lived on in the Vin K 350 truck motors until 1995. The truck engines never got roller lifters until the introduction of the Vortec L31 350 in 1996.

 

[supton]

Cylinders 4&6? Those are adjacent to each other. Any chance they are leaking into each other? and not leaking into crankcase nor coolant.

 

[dfarr67]

Truck roller lifters came later- but the blocks may or may not have been machined for the spider kit- and had flat lifters from factory. My '92 came machined and was converted years ago to oe type roller.