compression numbers

[dieselrulz]

I've read, and read, and read... so I have some background, but a couple questions.

For starters, 1989, F250, 7.3, zf5. 196k. It got to where i was cycling the plugs 3-4 times when it was 40ish degrees to start it, so I decided to replace the g.plugs. (i've replaced the controller and relay previously, when the issue was actually a melted harness... but i digress.) Since I was in there, I have been curious about the compression, and thus decided to test it. (with the truck off, parked on a steep hill, in 1st gear, it will slowly creep forward.) I also replaced the factory starter with a powermaster 9050, based on a handful of threads i've read here, and replaced the alternator because it was only charging when the rpms were over 1k. The truck sat for about a week while the parts were shipped in. That was when i did the test, which leads to question 1:

With the truck sitting for a week, cold, would compression expectation change? Should I retest warm/lubricated? I also only tested cyl 2, 6, 8, and 1, 5, 7. I couldn't get the tester threaded in and connected to #4, or #3 without removing the hard fuel lines, and i figure 6 numbers would give me a good feel, since there isn't any real issue how the truck runs. the numbers i got were 345, 345, 410, 390, 350, 390. (not sure if it matters, but those are listed in the same order as listed above.) I am of the impression that 400+ is considered "good," while 350 is considered "minimum?" (with 20% variation max)

the follow up question is, what do those compression numbers mean to me? I have owned the truck 11 years, and have noticed the power drop while towing, but it still gets the job done. I love the truck, and i'm not opposed to rebuild, but i also work for every dollar, so it won't be an easy choice...

newly registered, long time lurker. =P

 

[riotwarrior]

Welcome

Sounds ok however the comp test isnt complete...and a leakdown says is it valves or rings

Having seen a few bad guids and worn seats and valves I almost suspect pick up heads and rebuild em and drop em on...

As you work for yer dollars it aint free to do the work.

But my guess is just that a guess...could be rings too...

An overhaul kit with heads ready to go you could in theory do rings bearings gaskets over a weekend.

What are you hoping to achieve what is your budget....once open that can of worms sometimes spills over quickly

JM7.3CW

 

[dieselrulz]

I guess the first thing i'm trying to figure out is, how bad are the numbers i'm seeing? The truck runs good, and now with the new plugs and starter, starts up quickly.

in the long run, i'd like to get back to where I was 10 years and 80k miles ago, which was deciding if the truck was good to pull a minor incline in 5th, or downshift to 4th, while towing. Now, the decision is 4th, or 3rd. (4.10 rear, 7300lb trailer.) the other option is consider buying a different truck - but i tend to live by, "better the devil you know..."

 

[sjwelds]

What condition is your IP and injectors in? you sure you're not losing power there?

 

[dieselrulz]

good point. i skipped over that. I wanted to do the compression test to see if replacing the injectors was good money after good, or good money after bad. the injectors and pump are factory. I also have had a handful of chevy diesels, and have only rebuilt the IP when it started leaking. My IP is dry - but they can loose pump pressure?

Ultimately, if compression numbers are "acceptable," I will probably put new injectors in it, and test the pump? If the compression numbers say i'm living on borrowed time, it may change my end game...

 

[sjwelds]

If you've lurked as long as you say you have, you should have picked up by now that the injectors and IP are only good for around 125k miles before considered at the end of their service life. I'd almost bet that's where your power has gone.

 

[sjwelds]

MAKE SURE YOU READ THE HALL OF SHAME BEFORE BUYING INJECTION COMPONENTS!!!!!

 

[dieselrulz]

injection component hall of shame... off to search some more.

 

[sjwelds]

http://www.oilburners.net/forums/showthread.php?19805-The-Oilburners-hall-of-shame

 

[dieselrulz]

thanks for the link! I also found the article by mel/Agnem on fuel systems - which looks like real good info if i'm going to dive into injectors, and pump. I have some reading to do before figuring a next step.

 

[OLDBULL8]

the follow up question is, what do those compression numbers mean to me? I have owned the truck 11 years, and have noticed the power drop while towing, but it still gets the job done. I love the truck, and i'm not opposed to rebuild, but i also work for every dollar, so it won't be an easy choice...

No kidding, a 27 yr. old truck has lost power.

IP's only produce enough pressure to fire the injectors. Pressure = Resistance. New DB2 IP's are capable of 5000 PSI.

Your power loss is probably worn out injectors, not so much as the IP. The timing will deteriorate (retards) over time as the IP wears.

A new set of injectors matched and PSI set at 1850 and timed correctly (nominally 8.5* BTDC) would make a world of difference. If the IP is not capable of at least 3K PSI, then it's worn out and should be rebuilt.

Your compression check is acceptable with a cold engine. A cold Check should be, at least 5 revolutions of each Cyl. at 100 - 150 RPM.

As @riotwarrior said, valves play a big part in compression. The rings, not so much, there is plenty of oil on the cylinder walls due to the squirters to cool the pistons.

 

[icanfixall]

As already mentioned.. Welcome to the forum. I feel your compression numbers are fine. The numbers when you need a rebuild are arouns 250 so what you posted is fine. Depending on the tester seal on the glow plug seat. your low cylinder could very well be from a leak during testing. Best way to find if that happens is use some soapy water and look for bubbles. Don't use oil. The temp of the possible leaking compression can ignite it. Might not hurt you but it sure will frighten you with the torch like effect. Truely at the miles you posted on probable original injection pump and injectors I suggest you replace them both. Mel owns Conestoga Diesel and is very well respected around the world for his great products and VERY GOOD customer service..

 

[Thewespaul]

How would you go about testing what psi your pump is putting out?

 

[dieselrulz]

How would you go about testing what psi your pump is putting out?

Are you asking how that is done? or how *i* am thinking I will do it? honestly, I figured there was somewhere I could send the pump to get it tested.

I will likely try to get in touch with Mel, because I really want to know how to change injectors and possibly the pump, without changing the way the truck drives, or the mileage I get. (i get 17 in town; totally good with that.) It also has a fantastic throttle response. It really is only when tied to a load, that the power just ain't what it used to be. I have read about "g" code, and "bb" code, and i'm kind of thinking that i want to pull mine out to see what I currently have, and try to go with the same code again? (try to keep throttle/mileage the same? more power is good, but i don't want to sacrifice mileage to get it, my old power was great really, for an older idi/na diesel.)

 

[79jasper]

Code doesn't really matter that much. Long as they have equal pop pressures and a good pattern.

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