While observing the fuel trim readings you can see when the misfire occurrs , the ECM lowers the fuel to that side of the engine(LH) head. The ECM appears to be responding correctly to the info it is receiving from the O2 sensor. Your observation of determining if the MAF sensor is questionable should affect both fuel trim readings , but that isn't what's happening. There is no doubt something wrong in the intake on the LH head.
The intake plenum gaskets are a well known problem with these engines and since the ScanGauge cannot pick up which cylinder is misfiring , it could be multiple cyls.
The fact that it runs better when cold is explained by the pre-programmed open loop settings which are richer for when the engine is cold. As soon as closed loop is achieved the ECM now is responding to O2 sensor signals.
From what I can recall some smoke machine tests will not reveal poor sealing intake plenum gaskets.
I believe you are on the right track suspecting the intake air leaks which appear to be coming from the LH head side.
As stated previously, when disassembled the gaskets look fine , but they still are leaking.
By the way , your videos are great and you did a great job of showing the info, Thank you!!

 

THANK YOU FOR THE INFO, HANKY!! About a month ago I bought plenum and throttle body-to-manifold gaskets and I was planning to replace those ... but then I put that suspicion on hold when I got the big difference in O2 signals between the sides (which now I know was a programming issue with the scanner, not a sensor/wiring problem).

Before taking the engine apart to replace the gaskets, I would like to ask you whether the so-called "unlit propane" method to detect the leak location may be an option for my case..

I'm thinking that, because the vacuum should be higher, in absolute values, than the pressure a smoker puts out, it's more likely for the engine to suck in propane and speed up at idle, rather than allowing smoke to come out of a seal somewhere ... However, I'm a bit reluctant to release propane on a hot, running engine, the generator's slip rings or a relay can spark anytime, and one spark is all it takes ...

Again, to all of you who dedicated some of their time to help me out of this problem, THANK YOU!

Cheers,
Dan

 

Your reservations are well founded and from the info I have found , propane enrichment doesn't always work in this instance.
If you decide to replace the intake plenum and intake manifold gaskets , you must replace the intake manifold bolts with the insulators on them because once compressed they do not reseal well if used over.
If you had a cracked plastic intake manifold, propane or carb & choke cleaner would work there.

 

I just checked on the ECT sensor again ... when disconnected, the PCM reports CWT as -40 degrees (as you correctly stated), but the fan still starts on high speed. I'd say this behavior results from two independent conditions: no actual temperature reading makes the PCM to revert to a limp-mode value (coolant temperature -40) shown by the scanner, while the open circuit seen on the ECT sensor activates the fan on high speed to protect the engine against overheating while running without temperature feedback ... Extra cooling is a lot less harmful than no cooling, especially since the thermostat will cut off the coolant flow through the radiator if the engine temperature drops. Smart, really smart. You gotta love the Ford engineers for their work ... well, maybe less so when it came to intake manifold seals

Will post again once I get the gaskets checked and / or replaced. It's quite a bit of work involved, and the spare time is tight these days.

Also, Hanky, thank you for the advice with the manifold bolts! Do you think it is worth trying to re-torque them before taking the whole thing apart? The car, after all, is 17 years old, even if it has only 49kmiles on it. In time, things may become loose ... I have the service and the spec manuals, so I can look up the correct torques for the bolts, and verify on the engine.

Cheers,
cougar_fan

 

From my days of working on these, loose intake bolts was not a big problem. However, it's really easy to find an intake leak by just spraying carb cleaner or brake cleaner onto the intake in the area of the suspected leak, and if the car responds by smoothing out or even running rougher, there you are. If you're in a sporting mood, you can monitor o2 voltage while spraying. If it stays up high while spraying, you found it. Thanks for the reality check, Hanky. Sometimes, i forget the obvious.

 

Sometimes the ability to forget is a blessing, especially, when I think I'm right an I ain't!
You can tell I have fun with the English Language.
You guys are great to work with and I have learned a lot.
What are we gonna do when cougar_fan solves this problem?

 

Among the three of us, I was the one to learn the most, by far ... thank you kindly for all your help, it's a pleasure to learn from people like you!

And please don't panic, I'm not out of the woods yet

Cheers,
Dan

 

There you go greasemark ! It was indeed a pleasure to learn form both of you gentlemen.
Just in case you have a spare minute check out my story under the Falcon thread.
I laugh a little and cringe a little, but still manage to laugh about it.
Those were the days when vehicles were simple as compared to today's vehicles.

 

Actually, read I wrote under the Falcon thread. I had a neighbor that had 2 1962 falcon convertibles in the backyard, and I rebuilt the transmissions in them (the old 2 speed, hanky; I know you remember them) when I was 12. That was so long ago, and I remember thinking not long after that that my '67 Galaxie was really simple compared to a 88 cougar. And know, I think THOSE are the good old days.
I said it for years now: I've learned more about computers working on todays cars then i EVER wanted to know.
I'll get up and go into work even after this gets fixed. And I know it will. There are still good techs out there! Thanks Hanky and Dan.
I just don't know for how much longer...

mark

 

Definitely not out of the woods ... I bought some carb cleaner and tried to locate the darn leak ... to no avail. Nothing changed in the way the engine runs while I went around and sprayed the outside contour of the intake gasket, the spark plug holes, around injectors, all over the intake manifold. Nothing changed, nothing ...The only place where I could not spray cleaner was under the manifold, between the cylinder banks.

But I've a new development for you, my friends: very early in my posts I mentioned that the sputtering appeared after the car sat for a week in damp weather. Well, every time the car sits for a few days, the sputter upon the first start afterwards is at its worst. As I drive the car, things get smoother. If I keep driving it daily for a week or so, the sputter goes to its minimum - as seen on the movies I posted.

Hanky, you correctly pointed out that the LH bank runs a bit leaner than the other side. Well, please correct me if I'm wrong: say some cylinders in the left bank misfire - on a random basis - because of poor cables (the cables are the only part I didn't touch when I suspected the ignition). No spark means the gas goes unburnt (well, eventually burns in the cats) which means the oxygen content in the exhaust gases goes up ... right? The O2 sensors see this, so the PCM reduces the fuel trims on the side that spits unburnt gas ... now causing additional misfires due to the lean mixture, on top of the ignition hiccups. Could it be ... a freakin' spark plug cable? On this car, one bad cable can impact the main spark and/or the residual spark. Throw two or more such bad cables in the mix and what you get is a random stutter. Electrical insulation worsens with humidity, which would explain why the car runs crappy in humid weather or after sitting for a while, and improves as the engine "dries" up through regular driving.

See, if it were just a vacuum leak, what would have made it come and go? A hole is a hole ... but in my car, there are (admittedly brief ...) times when the engine runs like a charm, and the fuel trims go positive as they should. Today i stepped on it and it flew from underneath me ... I felt I live again. The magic lasted 20 seconds, but those were pure 200 HP screams I was used to hearing before she got sick

Please let me know what you think, and I'll post back with my findings. I have new cables and new spark plugs, and they're going on the car this weekend.

Thanks again for your help!

Cheers,
Dan