I'm looking at buying a '95 3/4 ton pickup with a 6.5. Owner says it won't run and has low compression(100psi) on all cylinders. He replaced the heads, but it didn't help. Anybody know how the compression could be this low?

timming chain can give low compression.

faulty compression tester could also do it, no. as for not running, if the timing is wrong no engine will run. Ed

For all cyl. to be that low it would have quit running long before it got that far. I agree with edzzed. Timing or comp.tester. As close as the valves run in these I'd think they would hit the pistons if it jumped time.

Get another compression tester and try it again.

100psi is way to low for a 6.5 to run. It rely s on compression to run, all Diesel's do.

When the PO changed out the heads, did he state how the cylinder walls looked? If not this would be a good question to ask.

Sure seems too be too much of a coincidence to all be 100 & on both banks. The good news could be that there is consistency across all cylinders .Check again with a known good tester that does Diesels.

I agree with Diesel-T. That is strange that its 100 psi across all cylinders. When i comp tested my truck, last summer, i had 400 psi on the cylinders i could get to(5 of 8 were 400 psi 6th was 390 psi) because of lack of daylight and needed the truck back on the road.

also has the truck ran at all after he replaced the gaskets? wonder if he used a torque wrench? if not that could say alittle something anyone agrees? lol i know it is a pita get all to torque specs but there is reason for it just my .01 worth

timing sounds like the culprit unless he blew all the rings at once(not a chance)

timming chain can give low compression.
Wouldn't the engine have to have like atleast 300k miles on it for the chain to be worn out?

You never know what previous mechanics could have done. I just pulled the timing cover off my 95ld and found most of the bolts were not put back in. If you buy the truck you should assume the engine is shot. Offer less than $500 unless the body is immaculate. And yes normally the timing chain should last to 300k. My 91 6.2 had over 250,000 on it and was still acceptable according to gm manual. But you never know what could have happened. Test compression with known good tester.

You never know what previous mechanics could have done. I just pulled the timing cover off my 95ld and found most of the bolts were not put back in. If you buy the truck you should assume the engine is shot. Offer less than $500 unless the body is immaculate. And yes normally the timing chain should last to 300k. My 91 6.2 had over 250,000 on it and was still acceptable according to gm manual. But you never know what could have happened. Test compression with known good tester.
I haven't seen the truck in person yet, but the guy says the truck came from Florida so rust won't be an issue. Only problem the body has is a dent on the passenger side front quarter panel. It only has 140,389 miles on it, so I guess your right. It could be mechanic error.

Wouldn't the engine have to have like atleast 300k miles on it for the chain to be worn out?
gmctd would have been here telling you it'll run like a new engine with the timing chain replaced at 100k miles. Ed

I haven't seen the truck in person yet, but the guy says the truck came from Florida so rust won't be an issue. Only problem the body has is a dent on the passenger side front quarter panel. It only has 140,389 miles on it, so I guess your right. It could be mechanic error.
if he had it parked near an ocean, rust will be an issue in florida. have you ever been in a boat in the salt water. even though your lips never get wet you'll taste salt on them:eek:. Ed

Only time I have seen extremely low compression like that was on a gasser that had like 10 psi in every cylinder (it still ran but very bad) turned out the pushrods were bent, and barely opening the valves. But since he said the heads were replaced, I assume they would check stuff like that. I agree with others on the bad compression tester.

As for the no start... no telling, I always try and check the basics (fuel, air, spark..uhh I mean compression).

a gasser won't run with only 10PSI, and if the valves barely open, you would still get compression its when the valves don't close that you lose compression, but anyways i believe you chevy i just think you got the number a little exxagerrated...;)

Just asked my brother if he remember the psi, it was actually 30psi. It was a 1968 Ford Heavy Duty 300 inline six. When I say very bad, I mean like it didn't have enough power to move off idle. Turns out the valves were seized (sitting for 35 years will do that) and when it eventually started with ether, it severely bent the pushrods. It was blowing flames back out the carburetor so I look at my brother and am like "what the heck is wrong with this thing?" turned off with the second biggest backfire I've ever experienced hahaha!
I guess I did exaggerate a little:rolleyes:

why did he pull then replace the cylinder heads? check with known good pressure gauge, if their close, somethings either functioning late or miss adjusted. did they try squirting oil into cylinders during compression test. as to no rust, i lived in florida for 10 years, the whole body surface will start to rust, not just floorpan/fender area, less likely if 15 miles from ocean/water. those daily rainshowers pickup salt and dump in rain. don't rust as quick but will rust. good luck, if you want i. not running ain't worth much.

Just asked my brother if he remember the psi, it was actually 30psi. It was a 1968 Ford Heavy Duty 300 inline six. When I say very bad, I mean like it didn't have enough power to move off idle. Turns out the valves were seized (sitting for 35 years will do that) and when it eventually started with ether, it severely bent the pushrods. It was blowing flames back out the carburetor so I look at my brother and am like "what the heck is wrong with this thing?" turned off with the second biggest backfire I've ever experienced hahaha!
I guess I did exaggerate a little:rolleyes:

It can happen. My old John Deere had 4.5 to 1 compresson stock, which was around 60 psi. It had a long stroke engine though.

Heres an update, the truck is running and on the road. I don't think the owner is going to sell it anymore but I'm fine with that. I'm still amazed that a 6.5 with only 140,000mi had a bad chain.

I bought a police car 1 time. It didnt have any miles to speak of on it but that was the most worn out pos I have ever seen. That car must have had a billon hours on the engine.
As for that truck there is no saying how often the oil went low or how long the thing was left running.

I bought a police car 1 time. It didnt have any miles to speak of on it but that was the most worn out pos I have ever seen. That car must have had a billon hours on the engine.
As for that truck there is no saying how often the oil went low or how long the thing was left running.
Very true, but I would think if it was poorly maintained, the rod bearings would have gone bad long before the timing chain.