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Go Back   Dodge Cummins Diesel Forum > 2nd Gen. Dodge Cummins 98.5-02 24V Forums > 98.5-02 Powertrain
98.5-02 Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission Ect..NO ADVERTISING .

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Old 12-06-2013, 02:18 PM   #1 (permalink)
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compression test

what dose the compression need to be on my 98.5 cummins,the shop i carryed it to said it was 345 lb is that to low? that was on 5 cylinders.he diddent check 6.
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Old 12-06-2013, 03:23 PM   #2 (permalink)
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what dose the compression need to be on my 98.5 cummins,the shop i carryed it to said it was 345 lb is that to low? that was on 5 cylinders.he diddent check 6.
You be safe for 350-375 lbs per cylinder Wade.
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Old 12-06-2013, 07:16 PM   #3 (permalink)
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#1 had 350,#2had 365,#3had 340,#4 had 320 he didn't check the last 2.so i reckon its time to tear it down.
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Old 12-06-2013, 07:20 PM   #4 (permalink)
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#1 had 350,#2had 365,#3had 340,#4 had 320 he didn't check the last 2.so i reckon its time to tear it down.
Does the truck run fine? If so why do a tear down? The numbers seem decent.
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Old 12-06-2013, 10:28 PM   #5 (permalink)
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#1 had 350,#2had 365,#3had 340,#4 had 320 he didn't check the last 2.so i reckon its time to tear it down.
Wade that's really not that bad to jump to a rebuild. BUT , something's going on with your truck I just don't know what. it very well could be compression issues too. I'd ask a diesel mechanic and see what he thinks. If you do pull the head and the cylinders look good which really don't mean anything by looks. All that work was for nothing. You've done a vp,ECM,all sensors and nothing changes. But, engines usually don't do the problems your having for nothing. Bring the truck to Nebraska and I'd help you out.
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Old 12-06-2013, 10:36 PM   #6 (permalink)
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Those are some different number there 320 isn't crazy low but the one is 350 or something so that's not good but you could also be dealing with a cylinder Head valve seat cracks couple different issues how many miles on the truck
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Old 12-06-2013, 11:49 PM   #7 (permalink)
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Valves may not be seating right in your head leaving a tiny gap pushing air back out your intake and exhaust valves. Piston rings may not be setting against the cylinder wall good. Cylinder rings could have shifted around your ring land and causing compression to easily get through your rings as in your rings are suppose to be staggered all the way around. If one of them shifted it could cause cylinder pressure to drop a tad bit but not a lot.
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Old 12-06-2013, 11:59 PM   #8 (permalink)
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i always thought the compression numbers didn't matter exactly (cold vs hot engine) it was just the differences in compression between cylinders that caused issues?
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Old 12-07-2013, 12:48 AM   #9 (permalink)
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You would want your compression ratio in all of your cylinders roughly the same whether its cold or hot. Most diesel trucks are 16:1 but Im not sure if thats how these trucks are or not. The crank is timed around 10 degrees ATDC because of mechanical help. If a cylinder doesn't have a good compression on the compression stroke then the powerstroke isn't going to be as great causing a drop in engine performance.
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Old 12-07-2013, 11:00 AM   #10 (permalink)
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Wade that's really not that bad to jump to a rebuild. BUT , something's going on with your truck I just don't know what. it very well could be compression issues too. I'd ask a diesel mechanic and see what he thinks. If you do pull the head and the cylinders look good which really don't mean anything by looks. All that work was for nothing. You've done a vp,ECM,all sensors and nothing changes. But, engines usually don't do the problems your having for nothing. Bring the truck to Nebraska and I'd help you out.
well i don't remember if i told yal or not but when my girl ran it out of fuel .they went and got fuel for it.they put it in and tried to start it.well some boy stopped to help and they said he poured diesel in the breather.i don't know how much or it might have been gas.so now i think it might have over revved when it cranked.you know how it is when your not there and don't know what happened or how the truck acted when it quit.its like chasing a blind hog here.
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Old 12-07-2013, 11:03 AM   #11 (permalink)
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Those are some different number there 320 isn't crazy low but the one is 350 or something so that's not good but you could also be dealing with a cylinder Head valve seat cracks couple different issues how many miles on the truck
278,000 miles,just broke in good for a cummins
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Old 12-07-2013, 11:58 AM   #12 (permalink)
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Does the truck run fine? If so why do a tear down? The numbers seem decent.
it runs like new when you start it on starting fluid
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