As part of my inspection process to put my 1986 6.9 IDI F250 back on the road was to do a compression test. The numbers I got were unbelievably low as I thought they should be in the 400 PSI range. They were 200, 200, 225, 200, 225, 200, 200, 200 from cylinder 1 to 8 which really shocked me. I have owned the truck since it was new and have always maintained it very well. I have never checked the compression before, so I don't know what it use to be. The truck only has 152,000 miles on it and it really has not seen much use as it has not been my daily driver for 15 years now, but have always kept it for a work truck and have not really used it at all for the past two years.

What I find really weird is that with this low of compression is that the engine runs absolutely great, starts instantly, and hardly uses any oil. I don't think this would be the case with this low of compression. What are all of your thoughts on this? From what I have read on researching this in this forum and others is the compression should be at least 375 PSI.

Thanks in advance for your thoughts on this.

 

That is weird with that low mileage. The Ford manual shows 440lbs for a fresh 6.9.

I think I'd try another compression gauge for a second opinion. Doesn't make sense you'd be missing half your compression.

 

Maybe your gauge is off or you didn't test it right.... But I would run engine cleaners through your oil maybe your compression numbers are low due too sticking piston rings from sitting

 

Try a second gauge for sure. They are all to close to each other to be all bad.

 

Thanks already for the input. That is my thoughts as well. I will get it retested. It was my local garage that tested it as they have a diesel tester however, they are not really familiar with the IDI's as they don't really work on diesels that much, so they did not have many thoughts on this. I just can't understand half of my compression having disappeared. What I am really wondering is if these readings are accurate has anyone ever heard of compression being that low and the engine still runs and runs good too?

 

Was this with a warm engine? I beleive the 450 psi is for warm / wet engines. Ive heard of 6.9s over 500

 

With that low of compression, I wouldn't think it would even start, so I think the guys testing it just don't know how to do it correctly.

 

Do a leak down test. I bet it's a valve thing. If it is a top end prob you can do a head refresh and that is fairly cheap. New valve stem seals and use some grinding compound on the seats and bam good compression again.

 

Im with bonanza, i have a very, very, veeeerrrry hard time believing that those are the actual compression numbers, not all 8 that low and that even.

 

I would pull it hard and test it again it's amazing what that will do. cleans the carbon off the valves frees the rings up. sitting around is bad for any vehical.
You want to load it so that you can still run the speed limit, but struggle and put a hundred miles on it, or start driving it like you stole it.
About once a year I have to hook the 26 stock trailer to my idit and pull it down the interstate. It's a 90 mile round trip for me to go to a bigger town. I notice a diffrence after, but it gets less every year as I rack up the miles.

 

Just as an update I am going to be getting another compression test done tomorrow. However, I have been doing some research to try and explain what is going on here if these numbers are accurate and just want to get some of your thoughts on symptoms and etc. I am really wondering if it could be burned exhaust valves causing such low compression? The reason I think this could be an issue is that I have been running a Banks Power Boost 4" exhaust on there for about the last 14 years to reduce back pressure, but have never installed a turbo. Since I began my in depth research into this I am reading that not having back pressure without a turbo can burn exhaust valves. What is your thoughts and experience on this? I replaced the exhaust way back when because I was reading how good it was to reduce back pressure on these engines, but that was before the internet went big time and of course there was not the diversity of opinions out there, and maybe I made a big mistake? I did notice a big increase in performance after installing it, but I did have an issue where I started blowing oil out the exhaust right after I installed it, but that went away years ago. What is a good set of symptoms of burned out exhaust valves? Of course would compression be so even if this was the case as some valves would likely be worse than others, and of couse since low compression is low compression would the engine run so well? What are your thoughts on this? I can only imagine that getting the valves done and the heads rebuilt would be very, very expensive, but I will go that route if this turns out to be the case.

 

Back pressure is emissions bs unless anyone can prove me otherwise so taking it away is good because it doesn't harbor heat. It is possible to have burnt valves but on all 8 is crazy. However getting your motor really hot can cause exhaust valve pitting with will make them unable to seal ie compression leaking out. Either way would affect compression and be grounds for head removal to rebuild or refresh.

 

Have you tried pulling it yet, pulling it will clean the cabon off the valves helping them to seal better. one other tick you could try before tearing it down. is take the rocker arms off, you need a soft face hammer, bock of wood or brass punch. You want to hit the top of the valve stems and that will shock the carbon off

 

Ya, I gave it a hard run on Saturday with a 120 mile round trip which included some grades and I threw a 1,000 pounds of sand in the back as well. It ran great. I don't know though if it improved my compression readings yet as I can't get into the shop for another test until tomorrow. I still find it weird that I could have that low compression and have it even on all cylinders. I have never heard of components failing evenly.

 

I believe the mild back pressure requirement is for gassers. There's guys here running straight pipes and there's no report of burnt valves that I've seen yet. (Probably burnt neighbors though.)


I doubt there's anything wrong with your truck, rather the issue is likely your mechanic or his equipment.