Compression testing M929A2 8.3 Cummins

[idbankrman]

Anybody done a compression test on Cummins 6cta8.3? Ive got a dump truck that is hard to start and puts out tons of white smoke while cranking. I thought it was just losing prime but thats not it. The smoke does not smell sweet. Its a bit acrid and diesel smelling. When the truck finally starts to light off I have to keep the starter going a bit before it has enough rpm to sustain itself. This engine shows about 1800 hours on tach. Once it first starts it has a slight miss until warming up. Has ok power. Oil is fine and there is no evidence anywhere of a blown head gasket. Im thinking it could just be tired so thats why Im considering the compression test route.

 

[WillWagner]

Sounds like low fuel pressure. You should have 25-35 at the inlet of the injection pump.

 

[idbankrman]

I Put a new lift pump on it and a new return fuel check valve but have not checked the fuel pressure. Is the best way to check it to install a T in the fuel line for a pressure gauge?

 

[WillWagner]

You can do that or drill and tap the inlet fitting at the injection pump for 1/8 pipe and install a fitting there. When it runs, take the oil filler off. If it tries to blow off when you get to the last of the threads there is an internal issue. If it just has small puffs of smoke, you should be gtg.

 

[74M35A2]

Could be sucking air on the tank lines. Have you replaced them?

Cummins does not not specify compression data, or even leak down data. They only assess engine health via crankcase blow-by measurement. I did buy an adapter to do compression and leak on my 8.3, but have not used them yet.

 

[WillWagner]

Sure they do, copied from QSOL

014-012 Engine Compression

Table of Contents

Measure

[HR][/HR]

Measure

TOC

It is very time-consuming and expensive to begin removing internal engine components to diagnose failures. A compression gauge, Part Number 3164627, and adapter, Part Number 3164840, can be used as an aid to check for proper functioning of the following components: SMALL | MEDIUM | LARGE Next Piston ring sealing Intake and exhaust valve sealing Cylinder head gasket sealing Cylinder head cracked Refer to the appropriate procedure in this section for replacement of failed components. SMALL | MEDIUM | LARGE Previous Next NOTE: Due to variables such as starter and battery conditions that affect engine cranking speed, it is difficult to establish an absolute value for compression pressure; however, the following values can be used as guidelines: New engine (cranking speed at 250 rpm) 2413 kPa [350 psi] Used engine (cranking speed at 250 rpm) 2068 kPa [300 psi] It is recommended that the compression pressure be checked on all cylinders and then compared. All cylinders should be within 690 kPa [100 psi] of each other. SMALL | MEDIUM | LARGE Previous Next Piston Ring Sealing If the compression is low but can be increased significantly by squirting oil into the cylinder, the cause of low compression is inadequate sealing between the rings and the cylinder walls. Refer to procedure 007-013 for piston ring replacement. SMALL | MEDIUM | LARGE Previous Next Intake and Exhaust Valve Sealing If the compression is low on one or more non-adjacent cylinders and the pressure can not be increased by oiling the rings, poor valve sealing is to be suspected. Refer to Procedure 007-009 for cylinder head replacement. SMALL | MEDIUM | LARGE Previous Next Valve leakage is often audible from the intake and exhaust manifold. SMALL | MEDIUM | LARGE Previous Next Cylinder Head Gasket Sealing If the compression is low on adjacent cylinders and the pressure can not be increased by oiling the rings, the cylinder head gasket is probably leaking between the cylinders. Refer to Procedure 007-009 for cylinder head gasket replacement. SMALL | MEDIUM | LARGE Previous Next NOTE: Low compression on a single cylinder can be caused by an external leak to a coolant passage. A leak to a coolant passage of this magnitude will also result in coolant in the cylinder. SMALL | MEDIUM | LARGE Previous Next WARNING Do not remove the pressure cap from a hot engine. Wait until the coolant temperature is below 50°C [120°F] before removing the pressure cap. Heated coolant spray or steam can cause personal injury. A compression leak to the coolant normally will be detected by loss of coolant as the coolant is blown from the cooling system. NOTE: Remove the drive belt from the water pump. Run the engine for 1 to 2 minutes and check for coolant being blown from the radiator by compression gases. SMALL | MEDIUM | LARGE Previous

Last Modified: 03-Sep-2002

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[WillWagner]

Leak down testing is good for spark fired but as far as I know there is no leak down for compression fired engines

 

[74M35A2]

Interesting, thanks. We could not find such information when working on an ISX and was told Cummins does not publish such info. What is the source of this info (what is a QSOL)?

 

[WillWagner]

That is correct. No compression testing in anything above the midrange engines. I don't even recall if it is done on any B,C or L series beyond 1998 with the exception of some off road engines, for Cummins anywho.

QSOL,
https://qsol2.cummins.com/info/index.html

 

[74M35A2]

Thanks for the info.

 

[idbankrman]

Thanks for all the great info. I did the test with the oil filler cap and there really is no blow by. So ill keep looking for a leak in the fuel supply.

 

[idbankrman]

Ordered a Torks banjo bolt with built in snubber and a schrader valve from dieselORings.com. Plan to get a tester that will plug into the schrader valve. I already installed an adjustable return fuel check valve from Torks but since it was set at 25psi for a P7100 pump not sure what pressure it produces on the MW pump. When using the hand primer I can hear fuel squeezing past it with just two strokes so I think it is at a pretty low setting.

 

[74M35A2]

Will produce the same pressure on either pump. Your OFV sounds correct, not low.

 

[idbankrman]

Ill have the pressure gauge set up next week and see what I'm getting.

 

[idbankrman]

Got the new banjo bolt with snubber and schrader valve installed. I used a fuel pressure gauge set from Harbor Freight. Its meant for gasoline vehicles so it may not survive long but it only cost $16. Anyway the pressure at idle is 22.5 psi and at 1800rpm it is 30psi. So I think this is correct and the lift pump is performing properly. I tried adjusting the return fuel check valve and ended up with it at the max pressure setting but it did not increase the pressure from its original setting.