Pistons in otherwise stock lower end

[andyvr4]

Hi,

My 91 VR4 has been smoking on deceleration and after eliminating pretty much everything else I'm 99% sure that it's got a broken oil ring. I've been trying to convince myself otherwise (replaced the heads and inspected the cylinder walls at the time which look absolutely perfect) as well as replacing the turbo's (they had 115k miles on them and a fair amount of shaft play so they needed replaced anyway), but it's still smoking on decel and a little at idle.

I'm looking at hopefully just replacing the piston's and rings and honing the block, but not actually boring it since everything else looks great (the motor still as 165 psi compression (+/- 5 psi depending on cylinder).

I'm wondering if you guys think I'll be ok just honing it and replacing the piston's and rings and if so what you would consider to be the best piston for an otherwise stock lower end.

I was looking at the CP piston's on ips's website and they look nice do you know if they produce a stock size that will fit the stock bore?

Thanks,
Andy

[green-lantern]

Sounds like a bad PCV valve or you have the rear vent plugged off or something.

[andyvr4]

Sounds like a bad PCV valve or you have the rear vent plugged off or something.
I don't think it's the PCV valves as I've had 3 different ones on over the last couple of months thinking that there was a very good chance that might be the case!

The rear vent might very well be a problem though because I'm not exactly sure what you're talking about, haha. Do you have any pictures?

Also one other thing to add to the equation all of the smoke (at least at idle) is coming from the rear bank. I've ran the car with the downpipe off the engine and all of the front bank is completely clean, all of the smoke is coming out of the rear!

[green-lantern]

I don't think it's the PCV valves as I've had 3 different ones on over the last couple of months thinking that there was a very good chance that might be the case!

The rear vent might very well be a problem though because I'm not exactly sure what you're talking about, haha. Do you have any pictures?

Also one other thing to add to the equation all of the smoke (at least at idle) is coming from the rear bank. I've ran the car with the downpipe off the engine and all of the front bank is completely clean, all of the smoke is coming out of the rear!

The rear vent is the hose that goes from the rear passenger side head to the bubble. I've seen people plug that off before or put some sort of obstruction in there (a small Krank Vent can do it). If you want to eliminate the possibility of gasses building up in the crankcase then pull the hose off on the drivers side that connects the heads together and take it for a run. If the smoke goes away you have some kind of blow by issue.

[andyvr4]

The rear vent is the hose that goes from the rear passenger side head to the bubble. I've seen people plug that off before or put some sort of obstruction in there (a small Krank Vent can do it). If you want to eliminate the possibility of gasses building up in the crankcase then pull the hose off on the drivers side that connects the heads together and take it for a run. If the smoke goes away you have some kind of blow by issue.
I guess I didn't know what it was called, haha. I actually haven't even had that hose or the one between the heads connected through out the testing to prevent crankcase pressure buildup.

I also went ahead and removed the pcv valve completely just to eliminate it out of the equation. The car seems to be smoking just the same with these removed as before.

[green-lantern]

I also went ahead and removed the pcv valve completely just to eliminate it out of the equation. The car seems to be smoking just the same with these removed as before.

When you removed the PCV valve what did you do with the hose that goes to the lower intake?

[JasonY]

You would be fine, stock rods are good to 600tq reliably.

Probably would want to get a forged crank in there too if you have it apart.

Jason

[green-lantern]

When you pulled the heads did you check the combustion chambers to see if they looked darker or had oil on them? What about the exhaust manifold? You should have seen oil or dark soot if one cylinder was burning oil.

[andyvr4]

When you removed the PCV valve what did you do with the hose that goes to the lower intake?
I put a cap on it to block it off so that no air could get pulled through.


You would be fine, stock rods are good to 600tq reliably.

Probably would want to get a forged crank in there too if you have it apart.

Jason
I'll definitely consider this, but I'm not sure how much I gain. I'm never really planning on more than around 15 psi of boost on the 16t turbo's, do you think that I gain enough to replace it?


When you pulled the heads did you check the combustion chambers to see if they looked darker or had oil on them? What about the exhaust manifold? You should have seen oil or dark soot if one cylinder was burning oil.
I did look at them and all 6 looked fairly normal (no discoloration), there were slight vertical scratch marks in the walls, but they where small enough that you couldn't feel them with your finger nails. Nothing that I would consider abnormal for a 115k miles motor.


I also just ran another compression test to see if anything's changed since I hadn't ran one in around 25 miles and the results came back like below.

1=162
2=175
3=165
4=165
5=167
6=163

The 175 one is the only one that stands out, but only because it's compression actually seems high. These numbers were all taken on a completely cold engine with nothing added to the cylinders (I.E. oil through the spark plug hole).

I'll be glad for any other information that you guys could give me!

Thanks for all the help

[JasonY]

Cast cranks are a funny thing, some break in half at stock power, some people run the shit out of their cars that dont even know they have one, without failure.

Its like a piece of mind thing and if you can get a good used one, not that hard of a hit in the wallet.

Jason

[speedy25]

Oil control on rings shows itself on accel. If it was your rings you would be leaving a cloud at every traffic light. A leakdown test will show ring condition better than anything else. Decel smoking is vacuum related. Since you put new heads on it shouldnt be VSS, and you have experimented with the PCV system. There is still something wrong. Keep at it. You'll figure it out.

-SP

[TurboSinceBirth]

I have a 2nd gen 4-bolt shortblock if you need anything out of let me know. I also have some 2nd gen heads too. It never hurts to have a motor to build so there's less downtime on the car if the motor in the car needs work. I would suggest running some sort of a PCV system to see if it still smokes. I would recommend a set of Krank vents. They work well. All of the PCV valves autopart stores sell are backwards so they don't work at all when you install them. OEM is the way to go.

[green-lantern]

Those compression numbers look pretty good. I'd hate for you to rebuild for no reason.


Oil control on rings shows itself on accel. If it was your rings you would be leaving a cloud at every traffic light. A leakdown test will show ring condition better than anything else. Decel smoking is vacuum related. Since you put new heads on it shouldnt be VSS, and you have experimented with the PCV system. There is still something wrong. Keep at it. You'll figure it out.

-SP

I agree with speedy, leak down test should be next before tearing down. I got to wonder about your oil return lines also. If you didn't have any dark cylinders I'd think you're still burning oil from turbos.

[andyvr4]

Those compression numbers look pretty good. I'd hate for you to rebuild for no reason.
That's my problem as well I really don't want to tear the motor down with compression numbers like it's showing and the fact that the smoke really isn't even at the right times for an oil ring.

At the advice of you guys and Matt with dynamic racing (who definitely knows a thing or to about these cars) I think I'm going to try putting a set of krank vents on and seeing if they fix the problem.

The smoke definitely seems consistent with the turbo's return lines not getting the oil back to the pan (I've already checked the return lines and even pulled the pan to make sure there weren't any obstructions in it) so Matt's thinking was that maybe it just needed more vacuum in the krank case to help suck the oil back away from the turbo's.

I'll update this thread once I get the krank vents and let you know if anything changes!

[green-lantern]

OK, sounds good, just make sure you get a large Krank Vent on the rear. A place sent me two small ones and it made my problem worse because the rear vent couldn't relieve pressure quick enough.

It’s hard to believe that’s your problem since you were running open vents. Definitely worth a shot though.

[FastOldGuy]

To me it sounds like your turbos are the problem not the block.

[andyvr4]

To me it sounds like your turbos are the problem not the block.
The rear turbo's been rebuilt 3 times now though by Matt's machine shop! I'm fairly confident it's not bad, but I do think it's somehow relate to the oil return from it. I'm really hopeful from what he's said that the krank vents will completely fix the issues.

[AdamVR4]

Are you using stock oil feed lines? If they're aftermarket, perhaps they're too large?

Why have you had the rear turbo rebuilt 3 times? How many miles did you get on each rebuild?

The OEM rear return line is basically vertical -- unless it was physically clogged (which would be shocking), it's probably not the problem.

Just to be explicitly clear, while you've been testing, you've had all 4 inlet ports on the cam covers open to the atmosphere and it still smoked?

If the krank vents don't work (large in back, small at the lower intake manifold), I would suspect the valve stem seal installation on the rear head.

And for what it's worth, my krank vents do an amazing job creating vacuum at idle. My system is well sealed (nothing special, just good gaskets, RTV (the right stuff) and hose clamps where applicable)... When I remove the oil drain plug it barely flows until I remove the fill cap :)

[andyvr4]

Are you using stock oil feed lines? If they're aftermarket, perhaps they're too large?
They're stock


Why have you had the rear turbo rebuilt 3 times? How many miles did you get on each rebuild?
Matt and me kept thinking that the smoke must be coming from a bad rebuild on the rear turbo (of which the first we're 99% was bad, it developed shaft play in almost no time, the second rebuild was partially my fault because I put a new adapter for the stock oil feed like on the turbo when it was rebuilt and this adapter was machined wrong causing a lot of restriction in the oil flow basically starving the turbo of oil, then this time we're 99% sure that it's correct, at least hopefully.


Just to be explicitly clear, while you've been testing, you've had all 4 inlet ports on the cam covers open to the atmosphere and it still smoked?
Yeap, all 4 of them have been opened


And for what it's worth, my krank vents do an amazing job creating vacuum at idle. My system is well sealed (nothing special, just good gaskets, RTV (the right stuff) and hose clamps where applicable)... When I remove the oil drain plug it barely flows until I remove the fill cap :)
I'm 99% sure after my testing that this is going to fix it. I just tried hooking the 2 inlet ports on the drivers side straight to one of the sources from the intake plenum and then blocked off the other to inlet ports on the passengers side completely and this all, but eliminated the smoke at idle!

Without any check valves I didn't want to try and drive the car though because of the risk of pressurizing the krank case, so I'll half to wait for the krank vents to 100% confirm that it's going to take care of the smoking.

[FastOldGuy]

Then your turbo itself is probably OK but its related to the turbo or vacuum IMHO.

I had the exact same issue. I had crank vents. I did a leak down and mine was OK. My turbos had very low miles on it. Turned out it was the rear turbo.

[green-lantern]

If the krank vents don't work (large in back, small at the lower intake manifold), I would suspect the valve stem seal installation on the rear head.



This is what I was thinking but saying you had the same results after you replaced the heads I assumed that couldn’t be the problem. I did however have VSS issues with rebuilt heads and it happened after I vented my crankcase to atmosphere (like yours). If you have the plenum off then go ahead and pull the rear valve cover and check to make sure the VSS are still seated. My machine shop didn’t seat mine well enough and the seals rode up on the valves. I assumed since I didn’t have the PCV system hooked up (no vacuum in the CC) that might have helped them unseat. You saying it got better when you hooked the vent hoses back up makes me think that it could be your issue. Anyway if you can pull the rear valve cover just take a flash light and look through the springs and see if they have popped up off the guides.

[andyvr4]

This is what I was thinking but saying you had the same results after you replaced the heads I assumed that couldn’t be the problem. I did however have VSS issues with rebuilt heads and it happened after I vented my crankcase to atmosphere (like yours). If you have the plenum off then go ahead and pull the rear valve cover and check to make sure the VSS are still seated. My machine shop didn’t seat mine well enough and the seals rode up on the valves. I assumed since I didn’t have the PCV system hooked up (no vacuum in the CC) that might have helped them unseat. You saying it got better when you hooked the vent hoses back up makes me think that it could be your issue. Anyway if you can pull the rear valve cover just take a flash light and look through the springs and see if they have popped up off the guides.

I'll definitely check the next time I have the valve covers off, but I've already replaced the VSS 3 times myself on the old heads and then had the heads rebuilt by IPS, while still seeing the smoke exactly the same through each change so I'm suspecting that they're not the problem. I also would have thought if they were the problem the car would have smoked on initial startup which I've never seen out of!

[green-lantern]

I also would have thought if they were the problem the car would have smoked on initial startup which I've never seen out of!

I thought that also but it always took mine about one minute to start smoking. I hate to say it but it is a possibility you do have a broken ringland or ring. I hope you don’t and I’d try everything possible to fix it before rebuilding. Let us know how the vents work out.

[AdamVR4]

Well I hope the KV's fix it because I'm out of ideas! haha

[green-lantern]

Well I hope the KV's fix it because I'm out of ideas! haha

:stupid:

Seems like he has tried everything before he posted anyway :lo5l:

Wait! Gremlins?

[FastOldGuy]

No its a VR4 not a Gremlin


http://www.toomanymornings.com/wp-content/uploads/2010/04/AMC-Gremlin.jpg

[green-lantern]

You make all that money being a doctor and now you want to be a comedian?

Stick to the prostate exams :p

:D

[andyvr4]

Well I hope the KV's fix it because I'm out of ideas! haha
Me to because I'm out of ideas as well, I've never had this much trouble figuring out what's wrong with a car! I'm thinking there's a very good chance it's going to fix it after seeing what it did when I applied straight vacuum to the krank case!

[andyvr4]

Just to update this thread, I got the krank vents installed tonight and all of my smoking has disappeared! It looks like Matt and you guys were right that I just needed a vacuum on the krank case!

[green-lantern]

Just to update this thread, I got the krank vents installed tonight and all of my smoking has disappeared! It looks like Matt and you guys were right that I just needed a vacuum on the krank case!


Great news man! Hell of a lot better than a rebuild huh?

[TurboSinceBirth]

Just to update this thread, I got the krank vents installed tonight and all of my smoking has disappeared! It looks like Matt and you guys were right that I just needed a vacuum on the krank case!

:bigthumb: They definitely suck!(Pun intended)

[andyvr4]

Great news man! Hell of a lot better than a rebuild huh?

haha, yeah, I'm just glad I didn't pull apart an engine that has absolutely nothing wrong with it and the 100 dollar vs several thousand dollar fix is a nice bonus as well!

[FastOldGuy]

Never trust a Doc that puts both his hands on your shoulders during a prostate exam.

[NOMIEZVR4]

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