7.3 idi compression test numbers and discussion

[stick_witch]

Hey guys! I just did a compression test on my truck while I have it torn apart. New to these motors and diesels in general so if anyone could shine some light on this topic that would be great.

Results:

1. 380psi @ 21 compression strokes
2. 380 @ 21
3. 340 @ 21
4. 340 @ 21
5. 380 @ 21
6. 295 @ 21
7. 440 @ 21
8. 420 @ 21

Yes, the compression test took 21 strokes to peak out... but the numbers they got, to me, don't seem that bad (except the 33% difference between cylinder 6 and 8.). Someone also told me that as long as they don't drop under 250 I'm fine. This was a cold test as well, because the truck is all apart right now. Batteries fully charged and on the charger the whole time.

What do you guys make of these numbers? Looks to me like a rebuild might be in my very near future. I just got the truck and have it torn apart right now to have the IP rebuilt... Think I can get her 20,000 more miles to 200,000 miles before a rebuild/swap? IP rebuild and engine rebuild is quite a bullet to bite... but beats a $50,000 loan on a new truck that you can't even work on imo. Plus these things are so darn fun!!

 

[IDIBRONCO]

I agree about the fun part. You should easily be able to get another 20,000 miles out of her. You could probably get more than that. Normally, I'd tell someone to find another engine to rebuild and then swap them out, but these may not be very readily available in Alaska.

 

[hacked89]

I responded to your other post on the same thing but I'll go into more detail here.

1. Not good on cylinder 6
2. Not good on 21 strokes, a motor is usually within 6-7 strokes before it tops out and not more than 10
4. Upload a video of the gauge while you do it if you have a smart phone
5. Do a leak down test if you have a tester
6. Doesn't matter that your motor is cold

Compression usually builds quick and even. Generalizations.. Leaking piston rings will build slow on the first stroke and take many strokes to get to the peak. Leaking valves will be low on the first stroke but stay low. You don't have adjacent low compression so probably not that.

This is my video if you want to see what it's suppose to look like. You can scroll past my explanations on how to hot wire start and grind the harbor freight adapter

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[IDIBRONCO]

I didn't notice the "cold engine" part before. Your lower numbers may be much higher when the engine's warm.

 

[stick_witch]

I agree about the fun part. You should easily be able to get another 20,000 miles out of her. You could probably get more than that. Normally, I'd tell someone to find another engine to rebuild and then swap them out, but these may not be very readily available in Alaska.

Thanks for the response. Yeah, not sure on the availability up here, but then again, these motors are in a ton of different rigs of that era so I'd imagine I could get my hands on one, but knowing us northerners here in AK and Canada, we love to run these old rigs straight into the ground, so finding one worth anything might be challenging... then transporting the darn thing. They're quite burly.

 

[stick_witch]

I responded to your other post on the same thing but I'll go into more detail here.

1. Not good on cylinder 6
2. Not good on 21 strokes, a motor is usually within 6-7 strokes before it tops out and not more than 10
3. Do a wet test on cylinder 6
4. Upload a video of the gauge while you do it if you have a smart phone
5. Do a leak down test if you have a tester
6. Doesn't matter that your motor is cold

Compression usually builds quick and even. Generalizations.. Leaking piston rings will build slow on the first stroke and take many strokes to get to the peak. Leaking valves will be low on the first stroke but stay low. You don't have adjacent low compression so probably not that.

This is my video if you want to see what it's suppose to look like. You can scroll past my explanations on how to hot wire start and grind the harbor freight adapter

xc_hide_links_from_guests_guests_error_hide_media

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Yeah I saw your response, thank you for the insight! Also, thanks for the very helpful video! Funny enough that was the video that showed up when I was looking for info on doing the test and turning over the motor without the ignition, it was very helpful! Super cool to bump into you here.

Yeah, 21 strokes seemed way way too high. It built pressure pretty steadily until it topped out, it was just very slow (some noticeably faster the first few strokes than other though) and they all took many strokes to get there. And by a wet test I'm guessing you mean adding a little bit of oil into the cylinder to test the rings, right? How much oil you usually put in? I'd hate to hydrolock the thing. My guess is that the rings are all worn and 6 is also having some valve issues. It's at 180,000 miles, never rebuilt and probably not babied like some of the IDI's here, also it's an Alaska truck so lotssss of cold starts, so I imagine the motor needs to come out regardless of whether its the bottom or top end causing my woes.

Btw, love the project you got going on! Hahaha that wiring seems likes a nightmare though, I have a headache just thinking about the tedious work that it must take to sort that all out. Kudos to you though! That things going to be a beast!

 

[hacked89]

Thanks man, welcome to oilburners.
BTW scratch what I said on the wet test. Come to think of it, it would be difficult to get oil into the cylinders and the motor may try to run on it even tho it's a few drops to a teaspoon.

Normally you would get the engine warm on these and do it dry in place of the wet test.

If you have access to a leak down tester or can borrow one you can test the valve theory.

Also, another technique is to record the number of the first stroke along with the peak number.
That can give you an idea on the rings.

If you have worn rings you would have blowby, which is kind of subjective to discuss because most people don't have the ford/rotunda tool to measure it.

But generally you can judge it by oil consumption and if it bogs when cold.

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[dgr]

@hacked89 is the rotunda tool a manometer? I'd be interested in the proper setup and usage instructions if anyone has them.

Before springing for a rebuild, I would pull the valve covers and inspect them for soot. I'd also crank the engine over with them off and compare the movement of the valves between cylinder 6 and 8.

 

[hacked89]

@hacked89 is the rotunda tool a manometer? I'd be interested in the proper setup and usage instructions if anyone has them.

Before springing for a rebuild, I would pull the valve covers and inspect them for soot. I'd also crank the engine over with them off and compare the movement of the valves between cylinder 6 and 8.

I can see if I can find the part # in the book.
I would probably make one like my edited picture attached with a manometer like you mentioned.
The reason you drill that 0.40" orafice is because when I did the math you would lift the fluid in the manometer further than the length of the tube scale.
With the 0.40" on a 7.3 ive read online it shouldn't lift more than 4" on the manometer.
The conversion I was doing was inches of lift on the manometer to LPM to CFM and then max CFM 0.2*peakTQ

You could also get a $600 digital blow by sensor..
NA IDI shouldn't exceed roughly 6.76 cfm
Turbo 7.76 cfm

Both stock form.

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[dgr]

That hole. Genius. I assume the setup is with the oil dipstick hole plugged and the cap on. Any idea what rpms it's tested at?

 

[hacked89]

That hole. Genius. I assume the setup is with the oil dipstick hole plugged and the cap on. Any idea what rpms it's tested at?

Yes plugged and capped. It should be 100% load, first gear, estimate around 2200RPM.

If it's difficult to see I wanted to call out near the CDR it must be greater than 0.40". I used the ">" sign.

The loading will be the challenge, probably have to use a dyno.
You can get 100% load in neutral but it's very brief, too brief for the precision of the test instrument setup we are talking about imo.

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[Thewespaul]

Also could use a pressure transducer in the glow plug hole, that would give you great data on how much each cylinder is contributing and sealing. I plan on doing something similar with the race truck, but the hardware isnt super cheap.

 

[stick_witch]

Thanks man, welcome to oilburners.
BTW scratch what I said on the wet test. Come to think of it, it would be difficult to get oil into the cylinders and the motor may try to run on it even tho it's a few drops to a teaspoon.

Normally you would get the engine warm on these and do it dry in place of the wet test.

If you have access to a leak down tester or can borrow one you can test the valve theory.

Also, another technique is to record the number of the first stroke along with the peak number.
That can give you an idea on the rings.

If you have worn rings you would have blowby, which is kind of subjective to discuss because most people don't have the ford/rotunda tool to measure it.

But generally you can judge it by oil consumption and if it bogs when cold.

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I don't have access to a leak down tester.
First stroke numbers, if I can recall correctly were somewhere in the ballpark of the 70-120psi. They seemed pretty low on some of them. The truck does have blow by, but not like billowing and puffing from the oil fill kinda blow by. It just comes out as a semi light, steady white mist, but what i've read about blowby in the idi's has been so subjective. The only way to know for sure would to convert to an RDT or do a manometer test like you mentioned.

Like I sorta mentioned, pretty new "budget" truck for me, and I'm already rebuilding the pump, redoing return lines, o-rings, and olives for the winter here, and sealing a bunch of leaks, etc. etc. So, a rebuild isn't even in the question for me for at least a few more years, this is just my truck for truck things and my sunday driver, my DD is a 2008 explorer with a v6 gasser... for now anyway. So, I'm just gonna throw some good oil in with some Auto RX and give it a good clean and hope for the best until I can find a motor or can afford to have this one rebuilt.

 

[stick_witch]

@hacked89 is the rotunda tool a manometer? I'd be interested in the proper setup and usage instructions if anyone has them.

Before springing for a rebuild, I would pull the valve covers and inspect them for soot. I'd also crank the engine over with them off and compare the movement of the valves between cylinder 6 and 8.

Hahaha yeah def not springing for a rebuild. No soot under the valve covers, but when I had them off because the previous owner had RTV'd the old, hardened and cracked gasket and was leaking oil everywhere, it appeared that they had been recently cleaned, so while he was in there he must have also cleaned it all out, so at least he did one thing right, but that wouldn't help if there was soot build up on the valves themselves. Sounded like the truck may have been sitting for awhile too, so could be having some issues with valves sticking. Hard to say, Im going to run auto rx in the oil this next change so that should help if there are issues with soot. Valve movement looked normal though.

 

[IDIBRONCO]

I say to just run it and see what it does. It may surprise you.