Very high compression- what's going on?

Hello everyone,

New poster here, looking for some comments from you knowledgeable guys, and ladies maybe. Thanks in advance. I have searched the forum for this problem, but couldn't find anything. Low compression yes, but crazy high, no.

About a year and a half ago I found a Burb in good condition. I knew it didn't start real good, as the seller had warmed it up before I arrived, and even then it didn't start great, but it had a lot of positives and I had a good feeling about it, especially since there were quite a bit of maintenance notes, so I went for it. Within a week the water pump, original one I believe, started leaking, at about 190K. So I parked it for almost a year, and in the meantime I bought all four books on the 6.5 from this website and the CD, good job Jim and thanks, and I read these books on my long work trips abroad, and eventually did the water pump upgrade and dual thermostat upgrade, with lots of new hoses, and it worked out real well. Now that I've learned more about the rig, I see that the two previous owners did some good upgrades to it, as listed in my signature below.

Now I decided to address the unsatisfactory starting. Plan was to change all the original good stuff, injectors, glow plugs, lift pump maybe, and so on. But I wanted to do a compression check first, since if that's bad, nothing will help. I wasn't expecting bad numbers because the rig drives so nice, good power, no smoke etc, doesn't burn much oil. I haven't done a compression check for ages, and did the mistake of doing it on a cold engine, since I took a lot of items off to be able to remove the glow pugs on the right side. The numbers were awful, I thought I might have to scrap the truck. Most were mid 200s with a couple at 300. Then I learned from the internet how important it is to do it on a warm engine, and the importance of good batteries and starter. So I charged up the batteries fully and changed the starter and jumped out of my skin when it started on half a turn of the key, I just wasn't expecting that! So I guess that was the main problem, the starter. But since I want to keep the truck for a long time, I thought I'd still do the compression check properly on a warm engine, just to know what I have. Coronavirus lock-down everywhere and Amazon not delivering quick, so had to get a diesel gauge from NAPA. Did a couple of cylinders and was getting crazy high readings, and I guess the 1000psi gauge couldn't handle the 750psi I was getting and failed. Took it back, told the guys there that my engine should be 500psi max, when new, yet this gauge was giving me 750psi and then failed, so it's obviously faulty. Got my refund and researched gauges and ordered a glycerin filled analog Mityvac unit, feels like a real quality tool, I like it a lot.

Yesterday and today was spent putting in some good stuff that I ordered, Bosch injectors, ACDelco glow plugs, heat sleeves, had the original turbocharger rebuilt. I was tempted to just put it all together and be done with it, but I convinced myself to do the compression check after all. So I put it all together minimally and warmed the engine up until the temp stopped rising, ran it for about 30 minutes, shut it off, and took the turbo off again to get to the area on the right side, took all the glow plugs out, and proceeded with the compression check with my quality new gauge. Holy Smoke Batman, I got some reading that I did not expect. Can anyone tell me what is going on ?

I started on the driver side and worked my way around clockwise. Cylinder 1-725psi, 3-715, 5-720, 7-870, 8-820, 6-810, 4-905, 2-840.

Did number 1 again, got 615, but felt batteries getting weak, so changed them out, as had some fresh ones ready. But now number 1 couldn't get pressure. Tried a few times, each time disconnecting and reconnecting the gauge, but no luck. Went onto number 3, got 715 again, then number 5, got 680 this time, then number 7, got 790 this time, lower numbers but by this time the engine had cooled down considerably. Tried number 1 again, this time got 720, so maybe before the gauge was not connected properly, I was pretty sure it was on OK, could it be something else?

Anyway, knowledgeable good people, I've made it a long story, but can anyone shed any light on what might be happening? I really am quite concerned. I feel my engine might blow up anytime.... I can add that although I'm quite new to diesels, I feel my work is good quality, I take forever doing things, I really like to get things done properly. All glow plugs were out, gauge was connected properly, I cranked it until the pressure stopped rising, on average that was 10 seconds cranking per cylinder, some 9, some 12. What in the world is going on.....?

P.S. Jim - thanks again for writing those books. For the next issue, may I suggest an article about how to correctly do a compression check. Such an important procedure and so many things to take into consideration.....

Welcome to TDP

Usually I would be happy with ##'s in the 450 psi range and with cyl pressures pretty equal across all 8

Been around a lot of these engines and never seen any pressures like you are talking about

Actually I think the engine has just got great rings....

10 seconds or more of pumping is plenty....

The fact that there are no LOW holes is the most important part.

Under operation the piston only gets one compression stroke to make pressure and then fuel is injected...ignited and then exhausted.....

Just an FYI

You do not need to remove the turbo on the right side.

Jack up the truck...remove the RH wheel/tire and then unbutton the rubber inner fender flap..

You can access the glow plugs right through the hole in the fender.... Also great for getting at the starter wires and the front starter STABILIZER BRACKET

#8 glow plug is the only nasty one and must be accessed from under the truck without tearing stuff off.

Welcome aboard!

The "normal" procedure for checking compression on the 6.2/6.5 is, batteries fully charged, engine temp. up to 140F+ (for the duration of the test, so start closer to full operating temp), all glow plugs out, fuel off (remove ESS connector from the injection pump), and read at 6 puffs (maximum gage needle jumps). Varying any of this skews the results, but almost always will show either low pressure, or greater departure (pressure spread) values. A "pass" is described as 350 PSI+ and all cylinders within a 10-15% maximum departure. Your values are curious, and I've never seen anything close to that during a proper test. Are the cylinders wet? Before beginning the test, I suggest clearing the cylinders by a couple long cranks with the fuel off and glow plugs out. It may not make a difference, but at least it will be eliminated. If you continue to get fuel spray while cranking with the plugs removed, it is still injecting fuel.

That said, if the engine seems to crank normally at 100-200 RPM under compression load (glow plugs installed), and 200+ RPM unloaded (gp's removed). Seeing compression that high with a 190 PSI departure would concern me, as well. Something is amiss. The combustion "chamber" is the piston (Ricardo bowl), in combination with the pre-cup, so if they're original design, it's simply not making sense. 6 "puffs" usually takes about 3 seconds, so 10 seconds is too long. I've cranked them that long, and longer, but only as a confirmation on a low-pressure engine. I don't know what it will be on an otherwise healthy engine. This may be normal. The only explanation that comes to mind, all else being equal and correct, is excessive sooting in the cylinders. I can't imagine a mechanical condition that would raise all cylinders that high.

Another possibility is you haven't taken away the fuel. Perhaps you have some combustion going on while testing. I would expect to see higher numbers than that, but if your starting compression is ~200 PSI, even on a cold engine, that would explain a lot, including hard starts, hot or cold. Doing the test on a warm engine won't necessarily show higher values, but will allow a test that shows lesser departure values as the tolerances will be closer across all cylinders.

Thanks for the replies, and so quickly too!

I think you've just hit the nail on the head, there was fuel coming in, lots of it, there was lots of misting around the engine while I was by the steering wheel turning the key. This is the reason I suggested at the end of my post to include a write up in the books next time on the correct way to do a compression check, many considerations. I found all I could on the internet before doing it, and I disconnected the fuel lift pump under the rig in the area of the driver's seat, and I disconnected the glow plug solenoid at the rear of the engine on the left side. First time I'm hearing that the ESS connector should be removed from the injection pump. What is an ESS connector by the way, what does ESS stand for, what does it look like and where can I find it? I'm heading to the truck this morning, maybe I'll do another compression check with the ESS connector removed, but probably not. I now know what the issue was and I feel all is OK, certainly the way it starts now with half a turn of the key, without smoke, I think all is good and I'll do a compression check down the road sometime. But if I do one, I'll let you know what transpired out of interest.

Thanks again for your knowledge, I really appreciate it, and if someone could post what and where the ESS connector is on the injection pump and what I have to do to get to it, I'd appreciate that too. I'll check here in a couple of hours when I get to the truck.

Thanks.

ESS = Engine Stop Solenoid (or FSS, on MFI's, F = Fuel). It's the uppermost electrical connector on the IP (either, EFI or MFI, not including the idle solenoid on MFI). It stops or allows fuel flow into the IP, under any circumstance (provided it's working properly). On an EFI engine, the ESS is the secondary method of controlling fuel. On MFI engines, it is the primary. The IP (Injection Pump) is fully capable of drawing fuel from the tank (it has an internal transfer pump), even when the LP (Lift Pump) is not operating, if the ESS is active/on. It is much less effective than the LP, but it will still do it.

If you had fuel in the cylinders during a compression test, hot or cold, I'm surprised you didn't see even higher pressures. With no glow plugs and a semi-warm or warm engine, it may not have enough heat to ignite the fuel to combustion or for a start, but would certainly present a condition of expansion, resulting in higher "compression" values.

If your engine starts as easily as you said, don't worry about the compression (I wouldn't), unless you have excessive smoke, an obvious miss or odd noise, or something. Starting is the first stop at judging the overall condition of the engine, specifically compression, timing, fueling, and starting system. Even an engine with a broken crank or other serious issues may start easily if the remainder is healthy.

Thanks again DmaxMaverick for the reply, I saw it just before I started working on the Burb earlier today. Like I thought I would, I persuaded myself to do another compression test, since I had easy access to everything and was all set up pretty much. I've had a very educational day, when it comes to compression testing on the 6.5.

I had in mind what you wrote, about disconnecting the ESS etc. Warmed up the engine for half an hour while I got ready, and got to work. Day started with more despair... Checked cylinders 1 and 3, and again they were giving me crazy high readings. Then remembered what you said about the cylinders being wet, so went on to 5 and 7 and returned back to 1 and 3 after the residual fuel had been expelled.

Here's what I got. 1-420, 3-400, 5-465, 7-460, 8-475, 6-480, 4-430, 2-440. Cylinders on the right side were done blind as I didn't see the gauge, so I cranked for seven seconds which is what it took the left side, in view, to come up to max pressure. Then I thought I'd do the low one again, number 3, but it came up to 400 again. I feel all the readings are pretty accurate.

So, I feel much better about things now, although from the lowest, 400, to the highest, 480, is more than the recommended 10 to 15%. Don't know if you or anyone have any comments about the readings, but I'd be interested to hear them.

Like I mentioned, truck starts up with half a turn of the key, no smoke, drives well, so I'm happy with things. Glad I did the compression check, while I was in the area putting in new injectors and glow plugs and a rebuilt turbo, and don't want to do the check again for a long time, I found it to be a real pain in the butt...

So thanks again for the information, I feel very lucky you responded to my post, and let me know if you have any comments on the readings. You or anyone else that is. Thanks to all.

Phew i'm tired from just reading this,.:)
Just pull the ecm fuse next time;)

Just unplugging the fuel shut off works well too...

Quote:

---

Originally Posted by Heliraf

Here's what I got. 1-420, 3-400, 5-465, 7-460, 8-475, 6-480, 4-430, 2-440. Cylinders on the right side were done blind as I didn't see the gauge, so I cranked for seven seconds which is what it took the left side, in view, to come up to max pressure. Then I thought I'd do the low one again, number 3, but it came up to 400 again. I feel all the readings are pretty accurate.

So, I feel much better about things now, although from the lowest, 400, to the highest, 480, is more than the recommended 10 to 15%. Don't know if you or anyone have any comments about the readings, but I'd be interested to hear them....

---

Welcome to the board, and thanks for your comments.

The GM service manuals state that the lowest acceptable compression pressure is 250-psi after 8 puffs on a warm engine with a minimum of 180-rpm cranking speed (obviously, cranking speed affects pressures). The lowest reading cylinder should not be lower than 80% of the highest reading cylinder. Your readings are OK. The high mile 6.5 engines I've tested averaged mostly in the 350-psi range, so your readings would indicate good piston ring sealing.

As I read through your notes here I thought, as others did, that there was a bit of combustion going on, which was skewing the pressures to the very high side. The GM service manuals state that the injection system fueling must be disabled to avoid damaging the compression pressure gauge.

Compression pressure tests are not something most people can do - that's why it wasn't included in our literature. I'll add it to the Troubleshooting & Repair Guide at the next update. Thanks for posting.

Jim