low compression blues
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4th Gear
Joined: Nov 2022
Posts: 4
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From: houston& New Orleans
low compression blues
i have a 2000 7.3 i took it to the shop today and got some bad news.The problem that i had was at around 70 mph the truck would shudder and it felt like your neck was going to break. the mech told me that it could be low compression in 1 or more cyls.the truck has 325000 miles and it starts fine with no smoke that puzzles me. He's going to do a compression test and let me know what the cyls show.i may be in the market for a good reman engine.i must also add that i've got some blowbye which i think is causing my oil leaks which was the reason i took the truck to him in the first place. part of me wants to tell him to put chewing gum and flex seal around the oil leak areas and we can do a flex seal commercial.what do you think?
Hotshot
Joined: Dec 2012
Posts: 11,266
Likes: 1,649
From: Las Cruces New Mexico
How bad is the blow by? If moves/lifts the oil cap when put up side down on the filler neck that's bad compression.
But from your description it's sounds like it could be something else, maybe electrical like a loose UVCH or fueling problem
But from your description it's sounds like it could be something else, maybe electrical like a loose UVCH or fueling problem
Lead Driver
Joined: Oct 2005
Posts: 6,129
Likes: 1,919
From: Ponca City, OK
What's the reasoning of low compression being the cause of such a specific issue? I would think low compression would present across a broader range of power band and through all gears. How well do you know and trust this mechanic and his knowledge of the 7.3? Do you have excessive oil consumption or signs/smells of burning oil? Engine issues aren't usually speed specific, rpm specific ok, but speed? This sounds like a steering/suspension issue to me.
UVCH are the Under Valve Cover Harnesses. They're the wiring that connect to the injectors and glow plugs under the valve covers.
UVCH are the Under Valve Cover Harnesses. They're the wiring that connect to the injectors and glow plugs under the valve covers.
It's a Van Gogh
Joined: Feb 2010
Posts: 7,795
Likes: 1,327
From: Elk Grove, CA
Very curious to hear the compression test results. Symptoms do not sound like a compression issue. There are shade tree ways to check compression btw.
Before agreeing to a $$$ new engine delay the mechanic and post up here.
Where is the oil leaking from?
Before agreeing to a $$$ new engine delay the mechanic and post up here.
Where is the oil leaking from?
Hotshot
Joined: Mar 2014
Posts: 10,750
Likes: 1,065
From: Douglassville, PA
I'm not buying it for one second. What you describe sounds more like a wheel out of balance than anything but maybe something with a drive shaft. As stated above, that simple oil cap test will tell you something about your compression.
CNCFAB
Joined: Apr 2020
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Butt-Head
Joined: Apr 2004
Posts: 9,406
Likes: 2,895
From: Reed Point, MT
A misfire, either from mechanical, electrical, or fuel issues is more likely to be felt at the point of torque converter clutch application at lower engine speeds which in the case of stock 2000 programming would be about 42-48 miles per hour at the 3-4 upshift. Conversely, it would be prominent in every gear in a manual but at lower engine speeds since RPM and boost pressure will mask a "dead" hole. It's not going to be something you feel only within a 3-4 MPH range.
Also, it would take a lot of cylinders working then not working then working again to have a severe bucking (break your neck feeling) you describe. Compression issues don't work like that. Either you have compression or you don't. Methinks you have a guy trying to take you to the wallet cleaners.
Also, it would take a lot of cylinders working then not working then working again to have a severe bucking (break your neck feeling) you describe. Compression issues don't work like that. Either you have compression or you don't. Methinks you have a guy trying to take you to the wallet cleaners.
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Thread Starter
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4th Gear
Joined: Nov 2022
Posts: 4
Likes: 0
From: houston& New Orleans
low compression
What's the reasoning of low compression being the cause of such a specific issue? I would think low compression would present across a broader range of power band and through all gears. How well do you know and trust this mechanic and his knowledge of the 7.3? Do you have excessive oil consumption or signs/smells of burning oil? Engine issues aren't usually speed specific, rpm specific ok, but speed? This sounds like a steering/suspension issue to me.
UVCH are the Under Valve Cover Harnesses. They're the wiring that connect to the injectors and glow plugs under the valve covers.
UVCH are the Under Valve Cover Harnesses. They're the wiring that connect to the injectors and glow plugs under the valve covers.
CNCFAB
Joined: Apr 2020
Posts: 4,874
Likes: 2,006
30 psi of boost is diddili squat, wont cause issue.
Your mechanic is wasting his time and your money doing a compression check, just let it idle and see if there is enough pressure coming out of the oil fill to blow up a rubber glove or push the cap off (lift it) when the fill cap is upside down.
Are you burning oil?
Where is your pan leak?
where is the vc leak?
These trucks are old and leaks are normal unless they are maintained to the point of not leaking.
Why go back to stock? A turbo wheel and a reasonable tune will hurt nothing.
How much does your mech work on 7.3? Going straight to a compression teat because of a vibration at 70-74mph and a few oil leaks seem %100 noobish
Your mechanic is wasting his time and your money doing a compression check, just let it idle and see if there is enough pressure coming out of the oil fill to blow up a rubber glove or push the cap off (lift it) when the fill cap is upside down.
Are you burning oil?
Where is your pan leak?
where is the vc leak?
These trucks are old and leaks are normal unless they are maintained to the point of not leaking.
Why go back to stock? A turbo wheel and a reasonable tune will hurt nothing.
How much does your mech work on 7.3? Going straight to a compression teat because of a vibration at 70-74mph and a few oil leaks seem %100 noobish
Lead Driver
Joined: Oct 2005
Posts: 6,129
Likes: 1,919
From: Ponca City, OK
A few oil leaks doesn't mean the engine is bad. She's just marking her territory so the lesser trucks know what's Hers. Also agree that unless those are some hot race tunes you're running they shouldn't hurt anything on an engine the is moderately maintained.
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4th Gear
Joined: Nov 2022
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From: houston& New Orleans
i'll be getting the truck back this week and i'll let u know what the status is.what puzzles me is that it always started good with no smoke and i don't smell unburnt fuel at the tail pipe.
Lead Driver
Joined: Oct 2005
Posts: 6,129
Likes: 1,919
From: Ponca City, OK
I doubt you have a compression issue, irregardless of what the mechanic may claim. I still can't track his/her thought process where a speed specific shudder equals low compression. Again I have to ask, how reputable is this mechanic and what experience do they have w/ the 7.3, or diesels in general, or automotive applications period?
Fleet Owner
Joined: Jun 2004
Posts: 25,090
Likes: 1,111
From: Rio Rico, AZ.
The operatiing principles of a diesel engine is compression provides the necessary heat for ignition. It will be worse at idle, not at highway speed. Rev the engine up, then you have more dynamic compression and the low compression issue is kind of covered up due to the speed of the Otto cycle being accomplished. A compression test will verify, but I'll bet the numbers will be ok.