Questions! ...Lots of Questions! ...Tune up time again...Nuts!

[1999montana]

Hello fellow Forum users,

Just started the process of figuring out why the engine check light came on and stayed on this week. First hint was a slight miss at idle. The engine runs fine off idle, pulls just fine at highway speeds. In neutral it feels like there is s light miss though.

Happened shortly after doing a Combustion Chamber Cleaning using aerosol chemical introduced through the PCV valve hose. Had been done once before about a year ago, each time it trips the engine check light, but it would go away after a few runs. This time it is on solid.

Not a vacuum leak. Since I don't own a code reader / scanner, and since it's time for a tune up anyway at 165,000 Kms, I figured I would approach it the 'old school' way.

First, pull the plugs and while pulling them replace them with new ones.

Seems easy enough, except for the rear or right bank plugs....

Got half way through that and still can't figure out how to reach over the engine down the right bank to place a socket on the plugs...tried removing the coil packs, associated wiring and the steel bracket that everything sits on, but one 13mm nut in behind the alternator stumped my array of tools and my agility. Don't own a deep 13mm and there is too much wiring in the area to get good access to the nut anyway.

Couldn't get the engine to roll forward enough even after disconnecting the air intake snorkel and anything that might prevent it from rolling forward.

Placed the trans in neutral and tugged on that engine for all we were worth. Got it to move maybe 3 inches, but if it was possible, it could have gone at least another 2 inches before the fans would get in the way. Tied it off, put the van back in park and chalked the rear wheels to prevent any movement.

But even at this angle it still wasn't enough to reach back there. Must be doing something wrong, but what?

Put it back together and cleaned the ground reference on each coil pack and the bolt that secures it to the plate.

BTW, what is that silver plate attached on the bottom of the plastic mounting cradle that the coil packs plug into?

Also noticed what looks like a square vacuum switch that shares the vacuum with the PCV valve. The PCV is attached to one end, then the is a 'T' fitting in the rocker cover, and a hose going off to this switch. Have no idea what it is. I think the wiring attached to the switch goes to the coil pack.

Tomorrow (while the Thanksgiving turkey is in the oven) I'll jack the van up and go from underneath, ...blindly of course.

My theory:

I have one injector that is 'dripping' and causing the rough idle. May even have an original plug in the right bank. Should show up in the plug pattern as a slightly richer plug or a plug that is fouling or breaking down. Suspect number 3 to be the culprit. Not even sure if the dealer replaced all 6 plugs the last time it was tuned up 80,000 Kms ago.

Before I go to the bother of replacing 6 injectors, I'll power flush the injectors using equipment my class A mechanic and business partner has access to. If I can't get it to run better, there is always the mechanic around the corner from me. I suppose it could always be another sensor that is going / gone too. The 'Stealership' will be the last resort.

Does anyone know if NAPA, Car Quest or any other similar store here in Ontario can and will read the codes in the ECM just as AutoZone does for free?

[dewaynep]

My first choice would be to have your Class A Mechanic read the code(s) for you before you go dumping a ton of money into it for a loose gas cap or something.

[1999montana]

Thanks for the reply.

Did all of the routine checks (loose gas cap, etc) prior to deciding that the plugs should come out for replacement. Finally removed all 6 plugs on Sunday and reinstalled new ones.

Was running half decent with a slight miss on Saturday night after removing and replacing the three front plugs.

This led me to believe that I still had not found the root cause and so removing the rear plugs seemed the only solution at that point. Buttoned it up and ran it out of the garage for the night and to think further.

Sunday morning:


B@#ch getting at number one and the boots were on pretty tight, like they had never been removed in 100,000 miles.

Here's what the plugs looked like:



Notice the significant difference between the bottom row, left bank or front of engine and the rear.

The porcelain electrode on Number one is bubbled and severally discoloured. While this could indicate a plug that was not properly torqued, (will not allow proper heat transfer) it could also indicate a plug that has never been removed either.

The outer porcelain is scorched on Number one:



Finally reconnected everything and restarted last night.

The simple act of removing very difficult plugs had this result.


* Severe miss on more than one cylinder.

Reason:

* Could be that the wires were damaged while removing, although I attempted to twist and pull at the same time.

* Any other thoughts would be appreciated.
.
.

Spark plug wiring diagrams available seem to be somewhat inaccurate, but the routing was checked twice and confirmed to be right.
.
.



* Indication of 'Front of Vehicle' should mean front of engine and in this case front should be where cylinders one and two are, RIGHT?

Traced wires from coil packs back to each cylinder...everything lines up. Fortunately the coil packs are numbered too, (although you can't see the numbers once installed - Brilliant GM!)

Guess, it's time for a scanner/reader now to see what is happening and a scope to see what is shorting to ground or arching.

All my fault, of course.

Based on what I have found as a root cause, anyone have comments concerning the possible shortcut the Tech may have taken when tuning it up 5 years ago????

Finally, any thoughts about how I might have made this worse?

[merc81]

Not sure if you have a book or what, but your diagram confused me. Here is my attempt at hand diagrams:


This is the view while standing in front of the radiator. Arrow points to passenger side. The smaller box in the rear indicates the coils and the numbers on that box are the plug numbers that should connect in that order.

I'm not sure what plate you are asking about in your post. The coils are bolted onto an aluminum plate that acts as a heat sink and also has the electronics required to fire the coils in sequence. That plate has two connectors on the right and one on the left. Mounted on the same two studs in the rear is a bracket that supports the EVAP solonoid and it has two vacuum lines on its right hand side--one from the purge tank down by the gas tank and one from the engine. It also has an electrical connector. The coil pack has a connection from the crank angle sensor that is just down below the rear exhaust manifold and that cable (two conductors) takes a lot of heat and abuse. It connects to the right side of the coil pack. Should it fail, the ecm can still run the engine somewhat, but the timing will be off and starting will be almost impossible--normally the ecm does not supply timing signals until after the engine starts. Startup is done only with the crank angle sensor.

In my opinion, your plugs look the way they do because nobody ever changed the rear plugs before. The front plugs are likely much newer than the rear. I think you have the firing order messed up somehow now, that's why things are much worse for you. Even if you had arcing going on somewhere, it would still fire the plug somewhat, and you would also hear that as a snap sound.
Double check your firing order before anything else.

John

[1999montana]

Hello John,

Thanks for your detailed reply. Your description confirms what I discovered while rummaging around the rear bank of the engine. I may have accidentally switched one of the coil packs when I disassembled and cleaned them (took the rust of the ground reference that the rear bolt passes through). And I may have accidentally broken a wire while trying to remove it too.

I bought new AC Delco wires and dielectric grease this morning and will check the position of the coil packs too. So that silver gizmo under the packs sandwiched between the plate and the coil packs is the electronic logic unit? OK, thanks for explaining that.

And the EVAP sensor / switch too. Too many wires hanging around down there too, considering the heat they must be subjected to on a long trip.

I know the diagram is mislabeled, but believe it or not, it is readily available on About.com as part of group of vehicle schematics.

A class A mechanic around the corner from me confirmed that in his opinion the rear plugs had never been replaced. This is a known shortcut that some (but not all) Techs will take to save time. The telling tale was that the torque to remove the rear plugs was considerably less than the front and the boots were nearly impossible to get off.

And if you think that taking you vehicle to a GM dealer ensures that the job will be done correctly, think again. My van was tuned up in late 2002 (shouldn't have needed to be tuned until late 2003, but due to a misfire, I took it in). The cost was about $500 which (among other things) included 6 new plugs, plus a power flush of the injectors.

Even if the shop foreman is overseeing the job, if the scope says that the engine is firing at close to peak on 6 cylinders after the front three are replaced, the Tech's job is done. Score 3 plugs and a good hour at Flat Rate for the Tech and the dealer.

I'll post my results after doing the checks and replacing the wires.

Fingers crossed.

[merc81]

If you want better access in the back, try this:
The chains hold up the body of the van while the floor jack lowers the sub frame. . .



Don't try this without checking out the correct proceedure first or you will break things you don't want to break. . .

Here is all the room you need:

[1999montana]

Hello John,

Nice BIG pictures...too big for this web page I might add, but good none-the-less... and your point is well taken...plenty of room for sure!

Finally got it all back together and attached each plug wire one by one with grease. Makes it easier to route the wires when you do it that way and allows them to fall 'naturally' the way they want to as well.

Coil packs were in sequence (A mirror used to see the backside of the coil helped to confirm this), so it must have been a wire or two. Double checked everything and then, turned the key.

Runs on 6 cylinders, but I need to reset the ECM. Someone told me you can do this by disconnecting the battery, but no luck with that.

It might go off on it's own, but would be nice to start with the memory clear and see if any new codes appear.

[1999montana]

...SUCCESS !...

John,

Running on 6 cylinders (like a swiss watch) and the engine check light checked out on its own...go figure!...

Thanks for all your help, much appreciated.

Bob

[merc81]

Great! Glad you were able to see it through. Lots of folks just give up when those back plugs won't come out.

Sorry about the pix, shouldn't have posted 'em anyway. I did try to make 'em smaller just now though.
The ecm will cancel the light after some preset number of driving cycles have elapsed without the code setting again. A driving cycle is a run from cold engine temp through hot temp and driving speeds that get up to 30mph or so I think. Then the cycle repeats next startup. Some codes go off quickly, some take a couple days.
John

[rkvons]

Congrats!. In the past, I have successfully changed the rear plugs by putting the fron on jack stands, laying on my back under the engine and getting them fro underneath. It's tedious, but you can kinda see what you are doing that way.

[rockwood84]

getting the back plugs and plug wires from the underneath is the only way.