View original topic: A pop, high oil temps, and a leak
TomC Sat Jul 03, 2021 6:44 am

My long weekend plans have been scuttled because of a high oil temperature condition and a leak in my bus's engine. The two problems may or may not be related, and I'm scratching my head to figure out how to solve them.

The bus is a '77 Westy with a '78 GE-series engine. The engine has hydraulic lifters, dual Weber 40IDF70 carburetors, and a 009 distributor. The oil is Castrol 20W-50 and is currently about 2000 miles old. Instrumentation includes an MGL Stratomaster monitoring 4 CHTs, OT, OP, and RPM.

A couple of weeks ago, I went to start the bus after it had been sitting for a few weeks. I danced on the gas pedal as usual, but maybe with a wee bit more enthusiasm than usual. While cranking, I heard a pop that sounded like one of the usual backfires and burps, but louder, loud enough that I noticed.

I did a quick walkaround and everything seemed normal, so I drove off to work. After parking I noticed a trail of oil drips leading from the back of the bus back in the direction I came from. The valve cover gaskets were sticking out from the covers, and the oil breather had been dislodged. Fortunately I had not driven far, and the engine had lost only a cup or so of oil. The level was still above minimum. I replaced the gaskets and reseated the breather, added some oil, and drove home. Over the next couple of days there were no drips under the car when it was parked and the oil level was stable. The oil breather did not vent to any unusual degree.

I have never seen a pressure event like that. What was it? What might it have done?

Last weekend the dog and I drove a couple of hours to camp in a favorite spot. Normally I can hum along at 65mph and the engine is as cool as a cucumber. If I have to climb a hill, the limiting factor is CHT and not oil temperature. This time at 50mph I was exceeding 230F when I usually stay under 200F. I was able to get to the destination, then home the next day by driving slowly enough to keep the OT at or under 230F. That meant about 50mph at 3100 rpm. OP varied between 26 psi and 31 psi. I noticed that when the OP was 31 psi, the OT would come down a couple of degrees, and at 26 psi the OT would climb a couple of degrees. When I got home little or no oil had been lost or consumed.

I should mention that in the few years that I have owned this bus, the engine has been a real jewel. I didn't rebuild it, but whoever did knew what they were doing. All the nuts and bolts that I've messed with were torqued to the right values, and I have found no evidence of shortcuts or cheap parts. It has never leaked a drop.

- Checked the thermostat and preheat flap over the oil cooler. Works great.
- Checked the relief valve spring and plunger. The spring was 59mm long and seemed pretty springy. I replaced it anyway. The symptoms did not change. The plunger fit well and moved smoothly.
- I have not checked for nests or obstructions over the oil cooler, but I will. It just doesn't feel like the problem.
- I was about to change the Napa Gold 1521 oil filter and replace the 20W-50 oil with Castrol 10W-30 when the following happened:

In preparation of changing the oil and filter, I started the engine and ran it at 1000-1200rpm until the OT reached about 100 deg. I shut off the engine and walked back to drain the oil when I saw an oblong puddle of oil about 6" x 8" approximately under the oil pump. I could not tell if the oil pump was the source of the leak, or if it came from somewhere else.

So much for camping.

So, what might have caused the high oil temperature condition? Why is there a leak now? What was that pop, and is it related to the oil temperature? Before I drop the engine and start hunting around, is there any other diagnostic I should perform?

German metalmania Sat Jul 03, 2021 7:11 am

Sounds like you had quite a bit of back pressure in the crankcase that built up, so much so, that it escaped by pushing out the valve cover gaskets and breather tank. As for the cause? Possibly a sticking pressure relieve valve.. Thatíll take a bit of investigation. As for that oil puddle, What I would do next before yanking the engine, is check the other parts of the engine where pressure would look to escape, such as the oil filter gasket, the front crank seal behind the fan housing, the oil cooler and oil cooler seals. Removing the fan housing unit is time consuming but very doableÖ. Muffler first, surrounding engine tin, fan, then housing and alternator (and harness) as one whole unit. From that point youíll get a better look at the things I listed to check. I just did all this yesterday, so I feel you frustration.

[email protected] Sat Jul 03, 2021 7:44 am

your backfire probably popped your oil breather etc.

could have popped the crank seal too, who knows. i have seen backfires blow every seal out of an engine before

rottenkid Sat Jul 03, 2021 9:33 am

Did you find a small plug from the oil gallery? The pressure could have popped it in behind the fan shroud.

SGKent Sat Jul 03, 2021 10:00 am

looks like it is in the oil cooler, oil filter mount area. Also check to see if the oil pressure gauge / light blew apart. You can see the stream of oil in your photo.

Starbucket Sat Jul 03, 2021 1:59 pm

They sell a dye kit with a black light flash light that you add the dye to your oil, run it a few min.s then shine the black light and you will see the leak source.

TomC Sat Jul 03, 2021 2:44 pm

Great suggestion to take off the fan shroud before dropping the engine. I've been doing it the hard way for years. :-)

So, how does a backfire pressurize the case? Seems like if the mixture burned in the intake manifold, the best it could do is pass through a cylinder into the exhaust and/or back through the carb(s).

The oil pressure gauge is okay. It's one of those brake-line-into-a-tee setups so that you can run the VDO pressure sensor and the original pressure switch. Everything still looks tight.

I've never heard of that oil dye kit, but it sounds interesting. I'll definitely keep it in mind.

This is as far as I got today:

It looks like the gallery plugs are intact. I can't tell if the rear seal area is leaking or not, but some oil appears to be coming from above it, which gives me hope that the seal is still good.

I'll get back to it tomorrow and finish removing the fan shroud. I'll keep you posted.

Wildthings Sat Jul 03, 2021 3:07 pm

On start up you had a combustible mixture in the crankcase and it blew up. Likely this started as a minor backfire into the intake, but that backfire via the crankcase ventilation ignited a much larger explosion in the crankcase. I am going to guess that one of your carbs leaked enough fuel after shutdown that a couple of tablespoons made it into the crankcase and then from sitting enough oxygen entered the case to make it into a bomb.

Not sure how all this relates to your oil pressure problem. Where is your oil temperature gauge installed? Maybe it was damaged by the explosion and is no longer reading the same?

As for your present heavy leak I would start by looking at the oil pressure switch and the plumbing for such. Other culprits could be the oil cooler or its seals, the pulley seal which might have been blown out by the explosion, or an oil galley plug. Oil galley plugs are one more part to the engine that do not like 20w50 oil as it can cause very high oil pressure behind the plugs on a cold start.

cmonSTART Sat Jul 03, 2021 3:39 pm

Out of curiosity do you notice it burning or consuming oil at all? Might be hard to tell with it leaking I guess.

airschooled Sat Jul 03, 2021 8:28 pm

Looks like your right carb is plumbed to the crankcase vent. An intake backfire on one of those cylinders could absolutely blow out some oil.

If you had solid lifters, Iíd tell you to djust your valves, paying special forensic attention to the right side of the engine. Deflated hydraulic lifters can aide in backfiring, so drive it more? :)

TomC Sat Jul 10, 2021 8:15 pm

I had some fun today taking off all the tail end bits to have a look for leaks and associated problems, as well as clues to the overheating trouble.

The leak turned out to be this little fella right here - bad washer.

With everything apart I was able to confirm:
- Seals are good
- Gallery plugs are intact
- Pushrod tubes are tight
- Thermostat and flap system work great
- Oil pump nuts are torqued properly
- Oil cooler is in great shape
- No critter nests anywhere

After I put everything back together, I replaced the oil filter with a new Napa Gold 1521, and swapped the 20W-50 oil for 10W-30. A lap around the neighborhood confirmed no more leaks. The dog hopped in and we went for a little highway ride to see if the oil and filter helped the heating issue at all. It didn't. :-(

Today's air temperature was maybe 15 degF cooler than the last ride, so it took a little longer to get to the high temps, but we got there.
Steady state oil conditions were something like this:

60mph, 3300-3400 rpm: OT: 220+ degF OP: 25 psi
45mph, 2600 rpm: OT: 210 degF OP: 15 psi
0mph, 800-900 rpm: 215 degF, OP: 5 psi

CHTs were fine and the dipstick felt normal.

This is all hotter and lower pressure than I'm used to, but I don't know what changed.

I plan to replace the relief valve and spring with new parts, just because. After that, I'm about out of ideas.
Where else can I look, what else can I try?

raygreenwood Sat Jul 10, 2021 8:26 pm

So...when you changed the gasket ring on the oil pressure relief valve plug...did the spring and plunger come out while you were working on it? If not...remove and inspect them .It could explain your low oil pressure. Ray

[email protected] Sat Jul 10, 2021 8:45 pm

what makes you think it's hot? i don't get worried until i see 240*

if your gauge is off that could be a problem. if it's VDO then it for sure is the problem

SGKent Sat Jul 10, 2021 9:54 pm

not saying I told you to look in that area before you pulled everything apart but ... on July 3 old sgk pointed to your leak.. call me a non-woke ass for pointing the leak out ... .

airschooled Sun Jul 11, 2021 10:08 am

The Type 4 has larger bearing surface areas than a Type 1, so donít compare your pressure numbers to most of what you read on here. You know what your engine smells like when itís running well on a warm day. Fill it with 20w50 and drive it to hell and back.


TomC Mon Jul 12, 2021 9:23 am

Thanks to all for your replies.

SGKent, yes! You called it. Kudos to you for spotting the leak in a dark fuzzy reduced-resolution photo as well!

I was resolved to take everything apart just to give it all a thorough inspection, so I don't mind that I could have fixed the leak in the matter of a minute with no disassembly required. :-)

raygreenwood: I did inspect the spring and plunger and they seem to operate smoothly without sticking. I had replaced the spring with another one from a parts can, just to see if there was a difference. There wasn't. I also plan to replace the spring and plunger with new parts, just on account of because, and because it's easy and relatively cheap.

Skills and Robbie: I will adjust my notion of "hot". :-) I will also check my temperature gauge against a calibrated thermocouple/display to make sure that the gauge and I are telling the truth.

My takeaways from the temperature issue are these:

- Don't necessarily trust your instruments or your measurements
- Hot, but the engine is happy? Quit yer whining and drive! :-)

wagohn Mon Jul 12, 2021 9:35 am

You can verify temps with one of these - $10 on Amazon. A bonus being you can also annoy the cat with it.

TomC Wed Jul 21, 2021 7:21 pm

It's all good. The engine is fine. It always was. I feel better now. :-)

So I checked the temperature instruments on the bus against an Omega thermocouple and display. The Omega devices were calibrated for use in testing nuclear power plants, so I trust them as a reference. :-)

The VDO 323-055 300 deg F sensor and the MGL E1 Stratomaster have a pretty linear offset of about 7-9 deg F - low! The actual oil temperatures are higher than what the instrument reads. By rights, that should make me feel more anxious.


I drove the bus 8-9 hours North to Vermont over the weekend on a pretty hot day. The oil temps rose like always, but I slowed down and they never topped 240F.

Coming home several days later in cooler weather, the oil temps behaved themselves and stayed lower longer, mostly in the 230s. Every time I checked the engine it was as happy as always.

I have been successfully recalibrated. :-)

Thanks again to all for your advice!

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