Smoking *after* overrun
Discussion
Hi, I'm after a bit of engine advice and was wondering if you could help. The car is an '85 MR2, with a red-top 4AGE.
There is a big hill near where I live, the sort where coming down, you stick it in 3rd, take your foot off the accelerator and coast down for maybe 2-3 minutes with the engine on the overrun at 3-4000 rpm. So far so good.
However - when I get to the bottom of the hill and go back on the accelerator, I get HUGE clouds of smoke coming out of the exhaust. It clears after a few seconds and then all is back to normal. It's like oil is being stored up somewhere on the overrun, then getting suddenly dumped into the engine when I open the throttle.
The breather pipe is clear, plus the engine has good compression and doesn't otherwise smoke.
Any ideas, anyone?
There is a big hill near where I live, the sort where coming down, you stick it in 3rd, take your foot off the accelerator and coast down for maybe 2-3 minutes with the engine on the overrun at 3-4000 rpm. So far so good.
However - when I get to the bottom of the hill and go back on the accelerator, I get HUGE clouds of smoke coming out of the exhaust. It clears after a few seconds and then all is back to normal. It's like oil is being stored up somewhere on the overrun, then getting suddenly dumped into the engine when I open the throttle.
The breather pipe is clear, plus the engine has good compression and doesn't otherwise smoke.
Any ideas, anyone?
Valve stem seals or valve guides or both. My understanding is that as you coast you have a relative low pressure in the cylinders which pulls oil past the seals/guides into the combustion chamber which then burns off as you accelerate again. My 944 did this, put new seals in after checking for play in the valves and it was fine. Didn't have to remove the head to do it, took a few hours and cost no more than 20 quid.
Edited by dern on Thursday 10th May 10:21
splatspeed said:
getsomeone to follow you
if you get blue smoke on deceleration sounds like valve guides maybe
if you get blue smoke on deceleration sounds like valve guides maybe
should have thought the rings are more likely. you could always have a compression test but tbh id just drive it anyway. to get it fixed will be so expensive i wouldnt bother
What he said ^^^^ Yup I would go with valve stem / guide wear - what happens is that the engine generates a high vacuum on overrun. If the valve stems or guides are worn, this high vacuum sucks oil from the valve gear into the combustion chamber - then when you press the accelerator it gets burnt off in a puff (or cloud depending how bad the wear is) of smoke.
Worn piston rings give slightly different symptoms - the smoke is there all the time the engine is under load, because of oil "blow-by" the piston rings and because the rings aren't properly scraping oil off the cylinders on each stroke.
Worn piston rings give slightly different symptoms - the smoke is there all the time the engine is under load, because of oil "blow-by" the piston rings and because the rings aren't properly scraping oil off the cylinders on each stroke.
dern said:
Valve stem seals or valve guides or both. My understanding is that as you coast you have a relative low pressure in the cylinders which pulls oil past the seals/guides into the combustion chamber which then burns off as you accelerate again. My 944 did this, put new seals in after checking for play in the valves and it was fine. Didn't have to remove the head to do it, took a few hours and cost no more than 20 quid.
Edited by dern on Thursday 10th May 10:21
I have your old 944
it still drives lovley.
all though its starting to smoke a bit on overrun at 117k.
im going to have the head off it in the winter.
b.johnson said:
I have your old 944
it still drives lovley.
Hello there it still drives lovley.
Did you buy it from me or from the guy who bought it from me?
Sorry to hear about the smoking. Did it all with the head on so maybe didn't get a proper feel as to how worn the guides were
. Edited by dern on Friday 11th May 15:35
dern said:
Hello there
Did you buy it from me or from the guy who bought it from me?
Sorry to hear about the smoking. Did it all with the head on so maybe didn't get a proper feel as to how worn the guides were.
Did you buy it from me or from the guy who bought it from me?
Sorry to hear about the smoking. Did it all with the head on so maybe didn't get a proper feel as to how worn the guides were
. Edited by dern on Friday 11th May 15:35
Hello
Im not sure, i think the guys name was Paul.
The paint needed a bit of TLC when i got it,
it got a bit of a knock last september.
www.pistonheads.co.uk/gassing/topic.asp?h=0&t=317057
b.johnson said:
Im not sure, i think the guys name was Paul.
That was it. Poor bloke had to put up with my 3 year old telling him not to buy the car when he came to pick it up because her daddy really loved it... I was late home and rescued him
b.johnson said:
The paint needed a bit of TLC when i got it
Really? It was pretty good when it went (edited to add: going to Paul I mean, not that it was unreliable
). b.johnson said:
Oh crap. Really good car that one. I loved the way it felt, you know? Had the best weighted steering of any car I've had except for the westfields. Just needed a bit more power hence the change to the v8 westfield. Thought it would be really good with an LS1 conversion
Edited by dern on Friday 11th May 21:19
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