`91 Lum 3.1 crank sens died on hwy, cyl 4 plug smashed, now no comp in cyl 4!

[Scott-Ohio]

Anyone have a good 3.1L T engine out there?! Please read the following and provide thoughts on the matter--thanks.

Got a call from my son on Sunday (Labor Day weekend) and said when going from asphalt to concrete bridge and back to asphalt (big bump!) on highway the engine died and he couldn't get it started again. So on Labor Day I rented a tow dolly and towed him 200 miles back to Ohio from just inside Mich where it died.

Good fuel pressure, but no spark. Found infinite/open resistance in wire crank pos sensor (CPS) connector at ICM and verified cable continuity to CPS was good so I pulled it and replaced CPS with new ($17)--busted plastic pieces in lower part of engine explained why oil was dark black after changing oil/filter only 300 miles earlier!! Drained oil and filter and replaced with new--FILTHY!

And, while I had engine tilted forward to reach down to replace CPS (a bit too tight from below for me!) I decided to replace back spark plugs too. All (1/3/5) looked pretty good. But when when I replaced the front 3 plugs next (2/4/6) I found #4 plug had bent electrode--looked BAD, not good, not good at all!

With new CPS I had great spark again, but when trying to start engine (nearly caught too with pedal to floor), white/gray smoke poured out from under hood--specifically from the oil dip stick tube where dipstick was apparently blown/popped up halfway out while cranking engine and smoke continued to puff out of dipstick tube for a minute or two after stopping to crank engine. Checked compression of cyl 4 (should've done as soon as I saw condition of #4 spark plug) and as suspected it registered a big, fat ZERO! Cyl 2 & 6 measured 125 & 150 psi respectively. Don't really care about back 3 cylinders at this point.

Car only has 99,500 miles and is in great shape otherwise and doubt I will replace engine at this point in car's life, but am trying to think positive thoughts (from the movie "The Fan" starring DeNiro) positive things happen to people who have positive thoughts! Yeah, I know, he played quite the WHACKO in that flick!!

I'm guessing when the crank pos sensor cracked up in crankcase going down the highway it hosed up the EST (elec spark timing) or still put out spark awhile longer and caused detonation which may have broke a valve which came between pistion 4 and spark plug 4 (smashed electrode) and probably put a gaping hole in piston 4 and now have no compression.

Sound logical? Any other thoughts? Thanks in advance...Scott

[jeffcoslacker]

Sounds dead on to me. Or debris from the sensor got caught in between the skirt and the cylinder wall, stressed the piston and cracked it.

[jeffcoslacker]

Wouldn't be that bad to pull the front head and replace the piston, if the cylinder wall isn't gouged.

[jeffcoslacker]

Was that motor ever opened up for head repair before? I've seen a few rebuilt heads that threw valve keepers and shot valves into cylinders.

[Kooterskkar]

I think you broke it. The piston I mean. It would take alot of detonation to crack a piston on a n/a engine. But I assume that the T next to the 3.1 in your post means turbo. With a turbo running even 10psi of boost it is alot easier to crack a piston. Id guess that the factory boost settings are set around 7 - 8 psi though to be safe. In the event that the vacuum hose blows off of the wastegate can you will get whats called overboost. Too much boost for the engine to handle safely. Which means you are also likely to get detonation. My guess is that the top of the piston decided to fly off.
Pull the head and check it out.

Did that oil have a storng gas odor?

[Scott-Ohio]

Originally Posted by jeffcoslacker

Wouldn't be that bad to pull the front head and replace the piston, if the cylinder wall isn't gouged.

Good point Jeff, didn't even think of it during all the despair--thanks for the thought!

Originally Posted by Kooterskkar

But I assume that the T next to the 3.1 in your post means turbo. My guess is that the top of the piston decided to fly off. Pull the head and check it out. Did that oil have a storng gas odor?

Sorry about that mislead on the T--I meant to say the "T" engine meaning GM engine code.

I should have mentioned that about 6-8 weeks earlier, my son had allowed the oil level to drop to 2 quarts, as opposed to the 5 qts it should have! That darn O-ring around the oil pump (where the distributor used to be on the older 2.8L engines) had caused the oil leak to get REAL bad, so bad that when I pulled that pump out, there weren't ANY signs of rubber/old O-ring at all! Although I've taught my son to keep a close eye on that oil pressure gauge, it never really dropped much even being that low on oil! And, he says, the dummy light never came on. But 3 qts later, when it finally reached the right level on dipstick, I realized it was time to replace that O-ring that same day. So now I'm wondering if maybe he spun a bearing awhile ago (or at least caused enough bearing damage) and with only local driving, didn't experience a major problem earlier? AMAZING if it lasted that long before dying! But then, during that road trip, there were 4 big (200 pound+) guys in the car with plenty of "supplies" in the trunk, cruising along Michigan highways where legal speed limit is 70, (prob cruisin' 80+ travling with the flow of traffic), but then was it coincidental that the crank sensor "broke" when hitting that bump and at approx the same time mechanical damage occurred in the upper part of #4 cylinder?! I'm sure he lost spark fairly quickly, but come on, losing half of that plastic casing around that little sensor probably didn't hurt the lower half THAT much! And now I'm thinking that young, BLACK oil might have just been very badly burnt oil caused in the bottom half of engine!

To answer Kooterskkar's question about strong smell of gas in oil that was drained, I would have to say that it did not seem overly strong to me.

Well guys, thanks for offering up your thoughts and please share any other ideas you might have on the matter.

[jeffcoslacker]

You know, I was looking at mine, and you can see #4's intake and exhaust springs through the oil fill hole. You try having someone crank it, and see if one is stuck? My sister's Corsica threw a stem keeper and launched a valve into the motor, I could see it through the fill hole.

[jeffcoslacker]

Cars do some wierd stuff, huh? Back in high school days, I had a 1972 Chevy Vega, every couple of weeks it would start to misfire, I'd pull the #3 plug, and the gap was smashed closed. I'd gap it, put it back in, and it would run great until the next time. It finally cracked that same piston on the highway. In the middle of nowhere, Nebraska. In -10 cold, that night. When it went, it blew out the dipstick, and the rubber plug style fill cap. I had to drive that thing all the way back to Denver like that, ended up just pulling that plug out to relieve the blow-by. Had to squeeze 2 quarts of 50 weight into her every 25 miles (not easy at -10 degrees) and smoke was pouring into the car through the steering column, firewall, and anywhere else it could get in. Had to keep the windows down to keep from dying of smoke inhalation. No heater core, had to bypass it the summer before. What a nightmare! Little bugger made it home though. Gotta give it props for that! They just can't run mortally wounded like that anymore. That's one thing I miss about old cars. The new ones can just blink out for some unseen reason, and there is no amount of ingenuity that's gonna get your butt home.

[Kooterskkar]

Sorry about that mislead on the T--I meant to say the "T" engine meaning GM engine code.

ahhh, ok. Well that makes it a little tougher.
When the distributor o-ring was leaking on my corsica there were several times when the oil would get almost three quarts low, mostly becasue I would forget to check it. When it got that low the oil pressure guage would drop when I went around curves. If I was sitting still it would hold steady.