Weak preformance from a chevy 350

timing-belts

#1

I have a '91 chevy 1500 with 297000 miles. It is running weak (so its got a few miles). It runs and starts fine. I have changed the oil every 3000 miles. I was going to have the motor swapped and the mechanic told me all I needed was a timing chain. He claimed these engines should have the TC changed every 80000 miles. I have never heard of this and have been working on my own cars for 30+ years. Most of them were GM cars. Anyone else every heard of this?


#2

Never heard of CHAIN replacement unless it was broken. Belts are usually 60K or so as you know. How would a chain replacement revive a weak, worn engine? I think you had the right idea with a motor swap.


#3

Well, timing chains do eventually need to be replaced, but not at 80K miles (more like 200-300K miles). The only effect that a “stretched” chain will have is on the cam timing.


#4

A chain does not have to changed every 80k miles and generally last the life of the car, especially with regular oil changes.

With a complaint of weak performance and a 279k miles engine, the first step should be a compression test.


#5

Thanks for the feedback, You may have something with this, as there is a little engine knock when the motor is under stress. That could be from cam timing. I may do the TC just to see the effects. The motor has been a good one and it may be worth the effort.


#6

Thanks, a compression should be a first step in doing anything serious to a motor with this kind of milage. But, the motor doesn’t use much oil and if the rings were going, I should see the effects out the exhaust, I think.


#7

Have you ever de-carbed the motor? That would be the first thing I would do. It’s cheap, easy and after 300k miles, you can bet there’s some carbon buildup.


#8

Timing chains do stretch, and I’m sure a chain with 279,000 miles on it has stretched a bit. However, I have never seen “weak performance” from a stretched chain. Maybe others can comment if they’ve witnessed such a weakness.

I do recall years ago some manufactures put plastic coatings on the teeth of the timing gears, to make the mechanism a little quieter. That plastic coating would wear and in some cases, cause the chain to jump a (worn) tooth.

One crude debug step you might be able to do is to see if your valves are opening and closing at the correct time. If you can find the specs for your engine, you should be able to find the number of degrees after TDC that your valves should open and close. Then, with a the valve covers off, and your #1 spark plug removed, and perhaps with a degree wheel, manually rotate your engine and watch to see if the valves open and close as spec’ed. We had to do this in the engines class I took years ago. I never did it in the field.

Again, others may have different thoughts on the above comments.

Joe


#9

That’s not necessarily true in all cases and compression is also affected by the valves and valve seats.