Well,

My Dad was neither a mechanic, or mechanically inclined. I am proud to say that I take after him.

He did leave me with one great answer to all befuddling issues mechanical.

He would walk away from the soon to be departed car/lawnmower/dishwasher shaking his head saying:

It's probably the oscillator...

 

I guess the rest of us are stuck, too. When I said checked for swapped sensors/ harnesses I didn't mean you should swap them, I just meant verify they are not swapped. But now we know they weren't. I would also try Brutal's idea of swapping upstream sensors side to side. Possibly a lean biased sensor on the left? The fuel trim reading and exhaust reading say that it really is lean, though. What happens to those readings if you add fuel? I'm just thinking out loud in print here.

 

Couldn't be a faulty throttle position sensor/throttle body issue i suppose?

Just guessing

 

If the engine was run with the valve seat only slightly off and the valve not fully closing for a while, the unburn fuel may have burnt out the Cat on that side only. A slightly gutted cat would exhibit reduce backpressure on that bank only. This may confuse the MAF slightly as it would try to compensate for both sides even though only one is the issue. Other systems affected as well maybe?? (a bit tired from 12hr midnight shifts, sorry for grammer) This could also be the cause of lean misfire on that side as well. Anyhow, I haven't sorted this out exactly but maybe it will strike a cord ... so to speak. Still lollying this issue around in the back of my mind & wishing you luck soon.
Cheers
TheCounty

 

It may be damage to the EGR piping during the rebuild.

 

just a couple to toss out there...since we're looking for either something telling the ECU that the left bank is running lean, or something actually IS letting in a ton of air on that side. May I ask what side the bent valve occurred? was it on this same side? If not, then it's not as likely the cat is toast. Have you read the voltage output of both sensors, up and downstream to compare?

Now, back in the engine, air leaking into the left bank...its not a rolled spark plug seal, as that would only cause a misfire on the subject cylinder, not random across the bank. What about the part load breather and its hose...might be worth a shot to swap that out for a fuel line, just to rule out a massive break in the 'ribbed-for-her-pleasure' part.

Isn't the vacuum brake booster supposed to be connected to the throttle body too?

This thread (though on a 4.2) has MANY similar symptoms, including a +19.5 LTFT on Bank 1, +9 on the other, turned out to be an exhaust leak at the manifold on the offending bank.

 

Well it has been a while, but I am back. Also the car is finally fixed.

I spent over 140 hours on this car. I have had it apart and back together so many times that on the final push to finish it, I had both cylinder heads off of the car in 2 hours and 15 minutes flat.

The issue, as I am sure you are all dieing to know, is the valve grind. The valves sitting lower in their seats left just enough change to one side, that the crankshaft sensor determined to engine speed decrease as a misfire.

The reason this was causing the FT values to go all over was the computer trying to compensate for this issue on the right, by changing FT values on the left.

Once I had the cylinder heads both machined, had all valves ground, and set all the clearances, when the car was reassembled there was no more misfire.

Hope this has been as much a learning experience for all of you as it has for me.

-Dan

 

Just a thought: Many years ago my wife and I were heading off to Texas for my Air Force tech training school...about 1800 miles. My brother-in-law had give us his 1962 Impala to use. It was a pretty nice coupe with a really strong 327 and 4 spd -- a darn cool car back then (1968).

Anyway, it was using some oil so we decided to have the engine rebuilt before we left and took it to a local country shop. After we picked it up and broke it in gently is ran like crap....nowhere near the power it had before.

The trip was a nightmare of oil leaking from the manifold and poor performance. We had to have the intake gasket replaced in Kingman, AZ. When we finally got to Wichita Falls, TX we had the engine totally torn down and rebuilt again. The mechanic found lots of shoddy work, but the biggest problem was the heads were warped. We couldn't figure out why, and had them shaved and magnafluxed to make sure they were right. There were no leaks after that but the performance still wasn't the same.

We drove the car back and left it with my brother-in-law. He had more problems with it. Eventually, he pulled the heads again and discovered they were not the original ones that came on the original engine. The first so-called mechanic had swapped his really good heads for a set of worn out, warped ones.

For this Jag problem...the possibility the heads are not totally matched to the block anymore would wreak havoc.

 

Does this mean that if you regrind any valves on one side, it's best to have all the valves reground on the both sides to even them up, in order to avoid the ecu trying to compensate for different compression outputs from each bank?

And would a new valve be thicker than a reground valve, meaning that a new valve should be reground to be the same as any other reground valves?

This is probably showing what a novice i am, but it can be useful to know these things about valves.

 

And may never see it again. Both my shop, and the dealer, have done one side before, and had no issues. Also the valve that was put in to replace the damaged one, was an oversized valve and seat, but this did not change the compression on this cylinder compared to the others on this side.

Overall, I have learned that doing both cylinder heads, though not cost effective, is the better way to go. Keeps you from spending 140 hours on a car like I did.

When in doubt, make them EQUAL.

 

Dan, so happy to hear you got to the root of the problem. Thanks for coming back to let us know what you found! All the best,

Steve

 

I'm having some difficulty in understanding the explanation of the cause of the misfire. Surely, regardless of how much the valves are ground in, or if new seats are fitted, once the valve clearances are checked and reset to restore them to spec, nothing could detect any unusual variation between the two banks?

 

The engine detects misfires by measuring crank acceleration. If the crank jerks at a certain location every time, it senses a misfire and it knows which cylinder corresonds to that crank angle. By shaving one head down and grinding metal off all the valves there, the compression change and valve actuation change created a jerking on that side by changing the crank acceleration, the computer detected it and acted accordingly.

 

Thanks for the update, that was a real head scratcher.
What genius hit on the solution?

 

Thanks from me too margate69, both for the solution of your problem, and the explanation of my question.
There are some very experienced mechanics on this forum, but your misfire puzzled everyone.

Good on ya