Need help with plug reading

[proporsche]

t77..in mechanics language "bone stock"--for me it is an original miles and engine no rebuilds and yes it matters;-)

Ivan

[proporsche]

bigfan911....let`s see how it will be with those new plugs..you say 1000 miles,right? Try to drive it a little harder as suggested by Kuehl i think?

Ivan

[kent olsen]

I might try taking the car out and run it hard. Red line pulls in 2nd and 3rd. Then pull the plugs and check them again.

[kuehl]

Rule out Intake leak theory:

Grab a can of brake cleaner (starting spray & fire extinguisher).
Start engine when it has been cold for a while (not running).
Let it warm up for 5 minutes.
Note your rpms.
Spray around the LH intake, watch for drop or rise in rpm.
Ditto RH intake (caution around the air filter plenum).
It would be nice to have a AFR gauge or CO gauge but rpms can tell you.

[T77911S]

Originally Posted by proporsche

t77..in mechanics language "bone stock"--for me it is an original miles and engine no rebuilds and yes it matters;-)

Ivan

you better get a new book

dealing with people on line (not in person) (probably in person too).
there meaning of "bone stock" and yours is NOT the same and THAT can leave you chasing you hind end.

[proporsche]

t77..never mind i just understood when he said bone stock...maybe you did not because you kept asking about rebuild .....again never mind, your technical comments do not help the OP;-) unless this is about English grammar;-DDD

Ivan

[T77911S]

check all the intake bolts and exhaust bolts.
do what kuehl said then try swapping injectors from left to right.
start with easy stuff.

[big911fan]

Thanks for all the input. Don't want to start any arguments here. For clarification, the engine has never been rebuilt. Original 45K miles since new.

I talked to a local mechanic and he has a course of action that isn't invasive at this time. He noticed that injectors 1-3 come out of the ecu from the same point. Injectors 4-6 come out from a different connection point in the ecu. The signals run to the injectors. The plan is to check the pulse width at the injectors to see if they meet specs (or see if left and right are the same at least)

First course of action will be to do a smoke test to see if there is a leak anywhere in the left bank.

Like someone said, "do the simple things (obvious things) first.

After looking at the plugs in person, as opposed to the pic you see, he said things didn't look all that bad but there is a difference. I think the pic magnifies the problem a bit.

[proporsche]

big..to be honest...the car runs great,right, no missing at all, no problems when you give a gas...after all no problem just you took the plugs out on a car which did 5 k in 6 years....
i would really say, what is the worry--drive it, man...
Ivan

[big911fan]

I hear you. That's my plan but still want to do the simple tests I listed. With the left bank running lean I just didn't want to risk burning a piston or something. Then again maybe it's a little OCD kicking in.

While it runs great it might even run better if the left bank gets all the fuel it needs. Also don't want to burn any valves. Maybe the difference is negligible however.

I realize this is not an emergency so won't get around to checking things for a month or so. I'll keep people posted if I find anything substantial.

With the cost of rebuids a little preventative maintenance never hurts.

[76FJ55]

As far as I know there is only 1 injector driver circuit in ECU, so if both banks are firing they will have the same pulse width. Pins 14 and 15 will be bridged inside the ECU to supply the ground signal to both banks of injectors.

[stlrj]

If my plugs looked like that, I certainly would have noticed. Mine would have sputtered just taking off unless I ran my throttle up to 3k and slipped my clutch like a torque converter.

[cabmandone]

My take:
I'd put 6 new plugs in
I'd put a torque wrench on the intake runner nuts because it's a known source for leaks
I'd do a few pulls with it then check the plugs.
Another easy test is a infrared temp sensor on each exhaust port. See if your left and right bank ports have about the same temp.
One more easy check: Pull your oil cap and see if the rpm's drop. If you hear a difference in the engine, you probably don't have any big leaks.

BTW, Sal suggested I put in a wideband o2 gauge which was some of the best advice I think I have received here. There's a lot of money in that engine compartment, why guess at what's going on?

[big911fan]

Here's kind of where I'm at after digesting all the suggestions.
1. There are new Bosch plugs in now. That's why I pulled the old ones.
2. If 76FJ55 is correct and there is a bridge to terminal 14 and 15 within the ecu it's probably not a fuel injection/ecu problem since all plugs are firing and the pulse widths should the same.
3. I'll do a smoke test before tightening the intake manifold bolts to see what's happening. I'd like to see if it's the intake manifold or not first. Otherwise I won't know if the problem is fixed for a long while. I did the oil cap test already. The engine does change rpm.
4. I can do the exhaust manifold IR test now. Easy enough to do.

[Quicksilver]

Reading the plugs will show you mostly what was happening when you shut off the engine. So I expect what we are seeing is after the engine slowly rolled up to the house with no load and then idled for a bit before it was shut off. Not that useful of a reading.

As to the unevenness of the plugs: At the age of these cars it would be common for there to be at least some small level of vacuum leak at idle that would make some cylinders lean and some rich.
- The Motronic uses "batch injection" which means all the injectors fire at once so depending on when the specific cylinder is in the induction cycle different cylinders will have different results from a port leak. So if the intake on a port opens during or right after the injector fires a vacuum leak on that port won't make much difference. The charge is right there and will get sucked into the cylinder.
But if a port has the injector fire right after the intake closes a vacuum leak has the whole rest of the cycle to draw the intake charge back up the runner into the plenum so some of the charge gets sucked into another cylinder.
In other words: At the high intake vacuum of idle your mixtures can be all over the map.

But at full throttle when the manifold vacuum is next to nothing any port vacuum leaks won't have any real effect.

- All this is well and good if you have leaky manifold gaskets.
__________________________________________________
But it all comes back to how to correctly read your plugs.
To correctly read plugs you need to cut the engine at a point in its operation where you want to see how it is running.
- That means under full throttle acceleration and generally in the middle of it's torque curve. Then you will get color on the plugs that really means something.
In a street car you need to be very careful that you don't turn the key and lock the steering column and kill yourself (or even worse, crash the car). You'll need to be able to coast to somewhere where you can do the plugs. If you are on the track you need to make sure you don't have someone run you over when you cut the power either.

You do this and then you will get something worth looking at on the plugs.

[cabmandone]

^^^^
Or you could throw plug reading right out the window and just go buy a wideband o2 setup and KNOW what your engine is doing at various points from idle, to tip in to WOT.
But yeah, if you want to know what it's doing at various stages, gotta kill it like quick said. Me, I just look for an overall picture. I want to see something on the chocolate milk end of the spectrum

But seriously, we have engines that are worth at least 10K... isn't an investment of a few hundred to know what the engine is doing a worthwhile thing?

[dhanl82]

I had M&K install an additional bung in the cat bypass that I purchased and then installed a wide band air fuel ratio gauge. It is without a doubt the best thing by far that I have added to my 82 SC. It is extremely helpful when tuning mixture and it provides a window into what the mixture and other related components are doing at all times/conditions. Now that I have it, I would feel blind without it!

Just my thoughts.

Dave

[ClickClickBoom]

Is the car parked on a side slope?

[ClickClickBoom]

https://www.motorcycleproject.com/text/reading_plugs.html

[big911fan]

No slope involved, car is parked in the garage when it's shut off.

If reading the plugs after a plug change is not ideal I think the next step has to be a smoke test to see if there's a vacuum leak. Probably long overdue on a 30 yr old vehicle anyway. Maybe next a valve adjustment. Last one was 6k miles ago. All just regular maintenance.

The idea of a wide band is very intriguing. Maybe in the future.

I live in an area of CA that doesn't require smog testing unless a vehicle is being transferred. Maybe I can get a "test" smog check that supposedly doesn't get reported to the state. Not sure if I believe that or not though. After all this is Kalifornia...