Blow-By & Compression Test

[Charles Allender]

I have a 1997 P32 6.5 TD Motor Home with unknown mileage. After returning from a trip with the last 5 miles under 35 mph, idling in the driveway produced considerable "Blow-By" coming from the pipe vent from the valve cover to beneath the engine.(No Codes)However, today when idling with a cold engine there was no sign of Blow-By. Does
this mean I have a crack (cylinder ?) which widens when heated ?

Although the unit is a 1997, I have a 4 Volume P32/P42 Chassis Front Engine Motor Home Service Manual for 1998 units (published by GM) . Under "Engine Compression Test" the manual says to remove the fuel solenoid driver fuse. The 1998 schematics shows the FSD being supplied by a 20 amp fuse called " ECM-1 ". Meanwhile the 1997 Owners Manual lists a 10 amp ECM-Bat fuse as protecting the PCM, Fuel Pump Relay & Oil Pressure Switch. I guess I can pull this fuse and if the engine fails to start I can try to check the compression ?

I would prefer having a garage work on things like this. I found a GMC Truck sales
shop with 5 full size bays (4 of them empty); however, when I asked about getting them to run a compression test, the SERVICE MANAGER looked me in the eye and asked what a compression test was. When I asked about getting the front wheel bearings greased he asked if they had zerks on them .

[ucdavis]

Chas,
Could be crack, but more probably a head gasket or hopefully some intermittent.
To run a comp test you'll wanna check #8 & 6 against #1 & 2 as a minimum which shouldn't be too hard if you can get a fitting to temporarily replace glow plugs (but given the symptom it would be best to check all eight. Injectors are way more complicated on the right bank than plugs to R&R.
Also, you can shut down the fsd by unplugging it (its easy to reach (relatively) on a Trek (I know ;) on the left side of the injection pump (IP). If that's too much, simply unplug the fuel shut off solenoid on front left of IP; way simpler than finding correct fuse where Safari hid them.
I'd also do a thorough check of the cooling system since overheating is the most likely source of a problem like this and fixing something w/out curing the overheat is money wasted.
Overheating is a simple matter of isolating 1 of the following (rough order of occurence IMO):
1) bugs. yearly water jetting might produce clean look to AC condenser & motor side of rad, but the space between condenser & rad can accumulate debris over the years that isn't seen till you loosen & tilt the condenser. Probably #1 cause of overheating.
2) bad coolant or coolant proportions; do hydrometer check & use distilled water
3) stats need changing; simple & cheap (except for the stoopid rad. drain location, oh well).
4) radiator corrosion over time = loss of capacity.
5) pinhole coolant leak in hose, head gasket, block or head crack, collapse of hose, etc.
6) low pump output due to accumulated corrosion or pump vane deterioration
7) overheating due to mods not respecting cooling needs or overstressing stock config w/repeated high GVW.
8) fan clutch failing (can't hear the roar no more)
With #3-6 the fan clutch may not engage much or at all.
The basic message from the Cracked Casting Club is "get dirty if you have to, but find that heat problem & solve it, & don't drive it till you do." If you worked thru above list every few years (#1 every year) & change all hoses/stats B4 trouble, it'd be cheap.

If Expansion Tank Does Not Return Coolant to Engine:
Cause is usually vacuum leak in system or system was opened (cap removed while hot) & not reclosed during cool-down. Since vacuum draw on coolant expansion tank is only way for coolant to be drawn back into engine, any air leak will cause engine to suck air instead of coolant from tank. Engine running hot w/no other cause (look everywhere for other causes first) could be air leak to coolant galleries.

[gmctd]

You did not mention over-heating symptoms, so the warm-engine blow-by is very probably normal. Heated oil gives off vapors, which adds to normal blow-by. Turbo'ed engine oil gets very hot, with the EGT at 700 or so degrees.
It is, however, ab-normal in the sense that it should be pulled back into the engine thru the CDR system, not vented to the street.
If this is normal crankcase venting procedure on the P-series chassis in the motorhomes, you may want to install the CDR and tubing.
You can still do the compression check. It's a good test on a vehicle with unknown service history, as are the coolant system checks as detailed by UCDAVIS.

jd

[ 10-14-2003, 04:04 AM: Message edited by: gmctd ]