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Our 91 blew up but now it's running again.

 
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haber



Joined: 14 Jun 2002
Posts: 1663
Location: Tennessee

PostPosted: Mon Apr 30, 2012 6:28 am    Post subject: Our 91 blew up but now it's running again. Reply with quote

This happened a couple weeks ago just before my wife and I left for vacation and this past Saturday finally had time to dig into it.

The 91 M20 broke a rocker. This engine is a 300k + motor and had a lot of wear on the cam journals. Probably why all this happened and I guess the clatter at idle should have been a sign. I had previously been unable to adjust the valve lash within spec and it was maybe .002" over a few thousand miles ago. I'm guessing the wear was accelerating at this point and it had got worse. The constant slapping of the cam just cracked the rocker where the pad is cast in.


The #6 intake rocker failed and jammed the camshaft. Here are the bits of it and look at the pad that runs on the cam lobe. It should be curved but not waved shaped? This is a lot of wear on the rocker as well.


This spun the cam gear on the end of the camshaft but there there is NO evidence of contact on #6 piston. That verifies the rocker failed first. However, #4 exhaust did make contact but it appears to be the only one. This surprised me that only one valve made contact but I'm happy with that. All the rest seam just fine. I've pulled just the one bent valve and the guides are a bit sloppy since you can wiggle the valve. I guess is .001-.002" clearance.


#4 piston took a hit but looks OK to me and should run fine as is I think. Maybe smooth down any high spots.


The cylinder walls have almost all the hone marks gone but there is no ridge at the TRR. The play is probably just over the .006" service recommendation but less than .010" Difficult to measure but maybe .008-.009" on the worst.

So I'm gathering opinions from the experts and building a parts list. I'm thinking we could get it running again with a minimum of work. Either a decent i cam and set of rockers plus a vlave or a lower mileage 885 head. The goal is just get a working car again, not something intended to be a keeper or even expect another 100k miles. Maybe just get another couple years (less than timing belt life) out of it while my daughter finishes up schooling, etc.

Does that sound like a plan or foolish if I didn't bother with valve grinding and guides if indicated by wear? Beyond that not much more than a new cam gear, timing belt, gaskets and head bolts would be needed I think. When checked about 4k miles ago the compression number we are about 155-165 psi. I just don't want to spend a bunch of money on a 300k car but since everything was working and it was a good little driver I like to try and fix it for a few hundred bucks.

Thoughts on sources for the parts we need?

Thanks for your opinions...
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Last edited by haber on Mon May 28, 2012 11:02 pm; edited 1 time in total
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tncean



Joined: 26 Feb 2005
Posts: 1652
Location: Chattanooga, Tn.

PostPosted: Mon Apr 30, 2012 7:25 am    Post subject: Reply with quote

Just my 2 cents, but a head rehab for guides, etc, at a machine shop isn't that pricy. Pretty sure far less than a "modern" monthly car payment, and well worth the peace of mind.

A person you might check with for parts is Wortle Smith at Bimmer Car Care in Bessemer, Al.

And, thanks for the "heads up" on clatter at idle and won't adjust.

tncean
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haber



Joined: 14 Jun 2002
Posts: 1663
Location: Tennessee

PostPosted: Mon Apr 30, 2012 12:51 pm    Post subject: Reply with quote

My son has found what looks like a good used head for about $100 plus shipping with a big crack in it where it appears to have been dropped. We would probably switch the cam and rockers and just run it. We will probably go this route since the goal is just to get it running again and maybe get a couple more years out of it. Thinking back about it, it was pretty noisy compared to any other M20 or M30 I've ever heard. I should have known it was a ticking bomb I guess. Now I do.

We were fortunate the failure happened closer to home than any farther away and I could get it back home to deal with it.
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85-325e



Joined: 30 Jun 2002
Posts: 4712
Location: Southampton, NY

PostPosted: Mon Apr 30, 2012 1:32 pm    Post subject: Reply with quote

Sorry to hear this Haber... The problem with the M20 engines is that they ALWAYS clack. It's pretty hard to know if the clacking is normal or a warning.

I hope you can get it back together pretty fast, and get it back on the road in a little while...

What a pain-in-the-butt.
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beejay



Joined: 18 Nov 2002
Posts: 1158
Location: Eastern, Pa

PostPosted: Tue May 01, 2012 8:26 am    Post subject: Reply with quote

I would think the easiest and cheapest would be to pick up a used motor and drop it in.
If it was me, I would be headed that way.
Too many miles, and too much metal bits in that motor. Makes me nervous.
Is the bottom end just a few miles away from a catastrophic failure? might be....
I would lay money on the oil pump and rod bearings look very bad.
I would hate to see you spend any money on head work only to have a rod let loose, spin a bearing or oil pump fail soon after.
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92 325ic gone Sad
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idanity



Joined: 21 May 2005
Posts: 3097
Location: N.W.U.S.A.

PostPosted: Tue May 01, 2012 8:36 pm    Post subject: Reply with quote

beejay wrote:
I would think the easiest and cheapest would be to pick up a used motor and drop it in.
If it was me, I would be headed that way.
Too many miles, and too much metal bits in that motor. Makes me nervous.
Is the bottom end just a few miles away from a catastrophic failure? might be....
I would lay money on the oil pump and rod bearings look very bad.
I would hate to see you spend any money on head work only to have a rod let loose, spin a bearing or oil pump fail soon after.



totaly aggree with that.
the m20's are pretty cheap these days, you may even be able to get a big 6, look for cars partin gout, many great engines just need a new car.
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robert1700



Joined: 04 Jul 2004
Posts: 1398
Location: Seattle

PostPosted: Wed May 02, 2012 11:05 am    Post subject: Reply with quote

I got a brand new crate M20 B25 if your interested.
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haber



Joined: 14 Jun 2002
Posts: 1663
Location: Tennessee

PostPosted: Thu May 03, 2012 6:56 am    Post subject: Reply with quote

Car is not a long term keeper. It's got damage to every panel and poor paint but at 300k miles still mostly working OK. We would just like to get it running again so it can be used occasionally for a couple more years. We are working on a used head where the cam and rockers still look good for very little $. Has not arrived yet but this will be the lowest cost and easiest repair we can make. Maybe it's a gamble but driving a car of this age always is as we already know.

Consider the only thing that got this car rolling again under it's own power was a collection of used parts in the first place.
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haber



Joined: 14 Jun 2002
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Location: Tennessee

PostPosted: Mon May 28, 2012 11:00 pm    Post subject: Reply with quote

The long weekend was what I needed and it's running again and soo quiet it's kind eerie. But now that the motor seems OK with some new and some old parts I have a question for you guys on i motor control. Who says these thing clack at idle. It's like a Singer sewing machine now, except...

First it seems to idle a little slower now and I don't understand this or maybe it's my imagination. Tach says about 650 once warm in Park. But when it first starts it's smooth in warm up mode but as soon as it goes closed loop it seems to almost miss now and then. Put it in gear (automatic) where it loads up and the computer steps the idle up and it smooths out again. Get underway and it's smooth as any e30 I've ever driven. And so quiet you can hear everything going on with the windows down.

I did try adjusting the idle air bypass in the AFM but it seems to make no difference and the computer just altered the mixture such that the O2 reading ended up exactly the same. Only while I was closing the screw and the computer had to adjust (I assume slightly rich) did it seem to smooth out. Once the computer caught up with me the same thing. Puzzled.

I adjusted the valves to .010 on the valve side which is enough to pass a .007 gauge on the cam side but not an .008. .010 cold is what I found in several sources although my Bentley does not cover the M20 i motor just the e from the e28 528. Is that too tight for an i and will a bit more open help the idle? My experience from e motors was always .010 but my 535i and M6 need to be .012. I plan to check lash again after a few hundred miles. Compression isn't perfect but with a pretty clean head and piston tops probably 140-150 range across all 6.

The only big dissapointment was getting it all back together to find that the AC expansion valve needs to be replaced. Going to order one and charge it up, again. Other than these things everything seems good to go and my wife and I drove around for about an hour today. There is a couple more maintenance things to do before my daughter comes home again in a couple weeks but by then I hope I think I trust this thing to head off again to DC, again.
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haber



Joined: 14 Jun 2002
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Location: Tennessee

PostPosted: Wed May 30, 2012 1:21 pm    Post subject: Reply with quote

I have been back to re-adjust the valves and the idle is much improved. Also took the AFM close to stoich as I could get it and it just runs as smooth as silk. I did my first adjustment of the lash on the bench before the head went on the car. After the tourqe down of the bolts and several thermal cycles the valvle lash was down to about .006". This is apparently much too tight and it will not run like it should. I did not imagine it could change that much but it did.

So all is now well but I thought I would let you know about this experience to wrap up this thread.
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beejay



Joined: 18 Nov 2002
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PostPosted: Wed May 30, 2012 2:37 pm    Post subject: Reply with quote

Good Job, Back on the road again. and good for another 100K.
I never thought of the I motor as noisy clacky either, but you can hear it clicking away when your in a bank drive through or something and you have the walls reflecting the sound.
I think it mostly injector noise as opposed to valves making noise.
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92 325ic gone Sad
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haber



Joined: 14 Jun 2002
Posts: 1663
Location: Tennessee

PostPosted: Thu May 31, 2012 8:53 am    Post subject: Reply with quote

I've been driving M30's too long now. They are a little noiser and you hear a little bit of valve clatter. They get set to .012".

In the M20 all you should really hear is injector noises. If you hear valves beware and adjust but if they are too tight don't expect it to idle right. .010".

On the other hand if you hear a whining noise that turns to an all out screem at high rpm then you might be driving an S14/S38. Cool .012" minimum (especially exhaust side) but valve train is so different it's not even comparable.
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kyshadetreebmw



Joined: 16 Apr 2003
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Location: Berea, Kentucky

PostPosted: Wed Sep 05, 2012 3:05 am    Post subject: Reply with quote

Reading through some of these posts, I keep looking for a "Like" button. Pretty Funny.
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