Blowby and rear main seal questions

[sholzy]

I have a 93 4.0 190K miles. I was driving around in stop and go traffic the
other day in this Texas heat w/AC on and my temp gage was reading much hotter
than normal. ( C_______^__H). Upon accelerating from a stop a little harder than
normal I notice a cloud of smoke I left behind. This is the first this truck
smoked so it was a bit horrifying to see that much smoke . Letting off
the gas the smoke eased up also. I pulled into a gas station to check the damage
and found oil dripping from the rear of the engine. My first though was rear
main seal.

After getting the truck home and not wanting to drop the tranny, I was reading
on this forum about a bad pcv valve causing excessive blowby and checking by
removing the oil filler cap and checking for smoke while the engine is running.
I checked that but no smoke and changed the 2 yr old valve anyway. I then
decided to warm the engine to its normal running temp (C__^_______H) and go for
a road test. I accelerated hard to 70. No smoke. Pulled over to look for
dripping oil like I saw before. None. Turned around and did the same thing. Same
results. I wasn't convinced I needed to pull the tranny and replace the seal.

Trailering the truck home I was thinking about a few things. I assume blowby is
greater at hard acceleration and highway speeds (say 60+) than at lower speeds
(say 30)? If my engine was running much hotter than normal the oil will be a
little ( or much?) thinner, and with the hard acceleration, would it be more
likely to push oil out the rear main seal compared to oil that was much cooler
and the same hard acceleration?

I don't want to yank the tranny so I'm hoping my thinking is on track and
I'll get reinforcement of that thought from you guys. Sorry for the long post
but I tried to give as much info as I could.

 

[Big Jim M]

That's alotta information

First off.. Did the smoke come from the tail pipe or was it oil leaking from the engine?
If it was leaking oil, perhaps it was from the back of the valve cover?
If it was truly from the rear mail seal and it doesn't smoke now then the pcv might have stopped the leaking. The blowby is caused by worn rings and the pcv takes that blowby out of the engine. Excessive blowby pressure PUSHES the oil past the rear seal.
As for oil being thinner...nope! Not until way beyond the capabilitys of the temp gauge. As long as the coolant is staying inside the engine the oil is not thinning out.
If the valve seals are getting loose, sitting in traffic for a while will cause smoke smoke from the tail pipe when starting back to speed again.
You might drive her till she is well up to temp and then do some stopping and going to see if the smoke cames back after a few tries.
Big JIm

I have a 93 4.0 190K miles. I was driving around in stop and go traffic the
other day in this Texas heat w/AC on and my temp gage was reading much hotter
than normal. ( C_______^__H). Upon accelerating from a stop a little harder than
normal I notice a cloud of smoke I left behind. This is the first this truck
smoked so it was a bit horrifying to see that much smoke . Letting off
the gas the smoke eased up also. I pulled into a gas station to check the damage
and found oil dripping from the rear of the engine. My first though was rear
main seal.

After getting the truck home and not wanting to drop the tranny, I was reading
on this forum about a bad pcv valve causing excessive blowby and checking by
removing the oil filler cap and checking for smoke while the engine is running.
I checked that but no smoke and changed the 2 yr old valve anyway. I then
decided to warm the engine to its normal running temp (C__^_______H) and go for
a road test. I accelerated hard to 70. No smoke. Pulled over to look for
dripping oil like I saw before. None. Turned around and did the same thing. Same
results. I wasn't convinced I needed to pull the tranny and replace the seal.

Trailering the truck home I was thinking about a few things. I assume blowby is
greater at hard acceleration and highway speeds (say 60+) than at lower speeds
(say 30)? If my engine was running much hotter than normal the oil will be a
little ( or much?) thinner, and with the hard acceleration, would it be more
likely to push oil out the rear main seal compared to oil that was much cooler
and the same hard acceleration?

I don't want to yank the tranny so I'm hoping my thinking is on track and
I'll get reinforcement of that thought from you guys. Sorry for the long post
but I tried to give as much info as I could.

 

[sholzy]

First off.. Did the smoke come from the tail pipe or was it oil leaking from the engine?
If it was leaking oil, perhaps it was from the back of the valve cover?
If it was truly from the rear mail seal and it doesn't smoke now then the pcv might have stopped the leaking. The blowby is caused by worn rings and the pcv takes that blowby out of the engine. Excessive blowby pressure PUSHES the oil past the rear seal.
As for oil being thinner...nope! Not until way beyond the capabilitys of the temp gauge. As long as the coolant is staying inside the engine the oil is not thinning out.
If the valve seals are getting loose, sitting in traffic for a while will cause smoke smoke from the tail pipe when starting back to speed again.
You might drive her till she is well up to temp and then do some stopping and going to see if the smoke cames back after a few tries.
Big JIm

Sorry, I forgot to say the smoke was from oil hitting the crossover pipe. The oil was definitely coming from between the engine and tranny. Top half of engine is oil free. Too much smoke for it to be valve seals. Since I couldn't get it to smoke again at it's normal running temp after changing the pcv valve, I just figured it was a combination of hot oil, worn rear seal and bad pcv allowing the blowby to push the oil out the rear. If i get in the mood I might yank the tranny and replace the seal anyway along with the tranny's front pump seal while its accessible.

It's been a good solid engine so far with only minor repairs like a new starter a few months ago, new head gaskets a year ago, new A/C clutch a few weeks ago, and a new fan and clutch this coming week. Not too bad for a freebie. It had 60k miles on it when I got it 7 yrs ago from my step dad and it still gets 22-24 mpg as long as I keep it below 65 otherwise it drops to 20 mpg.

Thanks, Jim for your info. This forum has been a good source of information over the past year for me.

 

[Big Jim M]

Don't trouble yourself with a new rear seal. Blowby will push oil past a new one just the same as a used one. If the pcv stopped the leak, make sure to keep another one handy for when that one goes away.
Big JIm

 

[sholzy]

Already have one. Picked it and a few other parts up while towing the truck home. It takes me a little longer to get motivated these days to work on something that takes more than a few minutes to fix.

Thanks again for your help.

 

[mkinttrim]

Was just wonderinng, did the old pvc valve still rattle? I always thought if it still rattled it was ok. Mine is doing the same thing, just had new cylinder heads put on, thought it was the valve cover gaskets but they seem to be ok.