1988 7MGE Misfire - Stumped!
 

Morning, all!

Back in October, I purchased a stock n/a 1988 Supra from a guy at work. The engine was missing badly and had a LOT of white smoke coming out the exhaust. Doing a Google search led me here, where a BHG was diagnosed fairly quickly.

I got lucky in the fact that this car was not his daily driver - he had only put about 20 miles on it since the symptoms started, so I drove it home (slowly) and noticed no overheating or weird gauge behavior, just the miss and the white smoke.

I checked the oil and it wasn't milky, and there was no oil in the coolant, just plumes of white smoke coming out, so everything appeared to be in good shape.

I tore it apart to do the head gasket job. Inspected everything - valves, block, head, etc. Couldn't see any cracks, bending, deformities, etc., so I cleaned it up and installed the new HG. New gaskets from HG on up.

While doing all this, I also replaced:

• Spark plugs (the wires looked OK, so I reused them. He said they were new)
• Distributor cap (the contacts looked a little worn - not bad, but I figured I'd replace it anyway)
• New rotor
• Ignition coil
• Fuel Filter

So I get the car back together, fire it up, and from time to time (more often than not, actually) it misfires through the whole RPM range. It has terrible loss of power when this happens, but other times the stars align correctly and the car runs just fine for an extended period of time.

I talked to the guy at the local parts store, and he seemed to think it might be a dirty fuel system component or water in the tank (because the car sat all winter) so I filled it up with fresh gas and put a can of Sea Foam through it. I've run through about 1/4 of that tank of gas and it isn't getting much better.

I jumped the computer to look for codes, and it isn't throwing any at this point, so I'm completely stumped as to what could be the problem. Sometimes it runs fine, sometimes it doesn't.

Advice on what this could be? Injector? Fuel pump? ICM? Sensor somewhere?

I'm completely stumped.

check your timing

Check for vacuum leaks

Check the harness connection to the CPS/Distributor.

You wouldn't happen to have a wideband?

Timing is set (per the manual) at 10 BTDC. I checked it when I first got the car back together, and then I checked it again after about 30 miles. Still right on.

As for the vacuum leaks, any specific places you'd check?

The distributor is a $15.99 Autozone brand - nothing special.

Good, so that's out.

If the hoses look remotely stock, I'd just replace them all... although this doesn't usually result in an intermittent problem.

The cap and rotor perhaps... but the distributor costs a bit more. ;) There are sensors inside the distributor body which detect the cam position so the ECU knows when to signal the igniter. A short, an open (∞), or too much resistance on that specific harness can cause timing issues that you won't see at lower RPM.


Then I would move on to testing the AFM and TPS when they are warm. Test the resistance across E2 and VS on the VAF while moving the vane inside. Test across E2 and VTA on the TPS while moving the throttle plate. The resistance should change with little to no spikes or sudden drops. HINT: Using a hair dryer to make sure the component you are testing is good and hot may help identify a problem you would otherwise miss.

While you're testing the AFM, test the FC switch. when the vane is closed there should be ∞ between E1 and FC and when the vane is held open there should be very little to no resistance across E1 and FC. Also test the IDL switch in the TPS.

VAF info:
MK3 TSRM On-Line

TPS info:
MK3 TSRM On-Line

I've read there are places that sell vacuum hose kits cut and sized to a particular model engine.

Any recommendations for a good place to get a hose kit?

Last I checked I found no kits for the 7M-GE. There are kits for the 7M-GTE, but they've got more of the larger diameter hose than needed and are several feet short of the smaller diameters.

This page of the EPC lists most of the hose lengths in millimeters:
Vacuum Piping

This page lists some of the others which are in both models and covers which sizes are used where pretty well:
Cygnus X1 Home Page

4mm and 3.5mm are the sizes you'll end up short on if you go with a GTE kit.

OK, did some more investigating after work tonight. I also replaced all the small vacuum hoses on the intake side of the engine. No change.

What I did find, however, was that it is cylinders 3 and 5 misfiring every time. I did the old "pull the wire off the distributor and see if the idle changes" trick, and I can remove both of those wires with no change at all in the idle.

Thoughts? What would connect these 2 cylinders?

3 and 5? Nothing... unless there's a problem in the ECU or the igniter which is causing a precise timing problem.

You might try checking for oil or water down in the plug galleys... that can sometimes cause a short to ground.

I know where the ECU is, but where do I find the igniter?

It's mounted right under the ignition coil. Aluminum box.

Ah...got it. I suppose that's a dealer only part? I found a used ECU on eBay, but igniters are probably pretty rare.

Have you heard anything about aftermarket coil/igniter packs? Accel makes one that's supposedly designed to replace both boxes, I think.

Or for a daily driver would you recommend just replacing the factory box itself?

I don't know of anything by Accel which would replace both. MSD makes an electronic ignition which would though.... but it's costly and not a direct replacement.

Don't just start buying parts because you think it *might* be one thing or another... you'll be broke long before you fix anything.

Have you checked the galleys for oil or water?

How old is the cap (not the rotor) on the distributor? If the contact for 3 and 5 have a lot of carbon buildup they can loose spark as well.

As for the igniter, typically when that goes it's not consistent like you've found... personally, I'd keep looking for something else which would be causing problems. The igniter and ECU are possibilities, but they're remote possibilities.

When I re-checked the plugs and wires I checked for fluids in the galleys. Cyl 2 had some, but I think it was from pulling the coolant hose off the intake and not having the rag in the right place to catch it all.

I even went so far as to switch the spark plugs that were in 3 and 5 to 1 and 2 to make sure it wasn't the plugs.

Nope...no change. Still 3 and 5 misfiring (or NOT firing, more accurately.)

The distributor cap is new. Just replaced it on Saturday to make sure that wasn't a problem.

Maybe I'll see if I can post a Youtube video of the engine idling or something. It's weird, because about 1/4 of the time it runs just fine, but I can't seem to tell what makes it kick in and out of that. It's not when it's warm, because sometimes it'll run fine from a cold start, and then kick in later with the misfiring.

I've been reading all day trying to figure out specifically what could be plugged/clogged/bad that would control the firing of the cylinders as opposed to an engine function like idling, and I'm coming up empty.

Have you tried different plug wires or tested the ones you're using? Are you sure the contact inside the boots is actually connecting to the cap's contacts and spark plug contacts?


Not dumbing it up... sometimes it's easy to get carried away and forget the little things.




Okay, what are all the possibilities? These: Coil, Igniter, IGt wire from the ECU to the igniter, CPS (cam position sensor in the distributor providing inaccurate info), bad wires, bad dist. cap, bad rotor, poor connections in the spark plug wires.

A bad ECU, or CPS should give error codes pretty much every time. A bad igniter may not, but it shouldn't be so perfectly off.... but I could be wrong. A bad rotor usually causes weak spark and intermittent misfires on ALL cylinders. A bad cap CAN cause what you're seeing depending on what's wrong with it. Bad wires or poor connections can definitely cause what you're seeing. Spark plugs not properly grounded (not terribly common) can cause similar problems too.

I would start with this: unplug the wires and on both sides push the wire into the boot so the contacts are much further forward and are sure to make good contact. When you reinstall them push the boot hard enough that once the clip connects the boot will slide forward over the spark plug and over the cap's outlet. NEVER remove spark plug wires by pulling on the wire!

Then, failing that, pull the cap and rotor. Clean all contact surfaces thoroughly with a brass wire brush.

If still no good, try installing a better ground in the igniter: IGNITER GROUND... crispics - SupraMania (That's for the GTE igniter, but I've been told the GE igniter is similar and just as poorly grounded.)

Heh...you're right, of course. The simplest solution is often the correct one.

I'll take all the wires out of the harness and see if maybe I can switch 2 and 3 or something - get one to stretch to the other plug hole, just to see if I get a spark there.

The cables LOOK OK, but that doesn't really mean much. Other than that, I don't really have a way to test them.

Of course I'm right.... :rolleyes:

I edited my last post.

Yeah, looks don't say a thing for plug wires. Try swapping 2 and 3... I think they're about the same length, same for 4 and 5... or was it 5 and 6?



EDIT: Oh, and BUY A MULTIMETER!!! Even a cheap one.... you're going to need it. Actually I can't believe you've been trying this without one.

Oh, I have a multimeter around here somewhere. Problem is, I just moved not too long ago and most of the non-essentials are still in a box somewhere.

I unpacked all my tools, though.:)

I'll see if I can find it.

I'll report back. Thanks for the help!

OK...here's the update
 

Cleaned all the contacts in the distributor and re-grounded the igniter per the instructions in your post.

No change.

Changed spark plug wire 2 to the 3rd cylinder and vise versa.

No change.

I can pull the plug wire off at the distributor cap and see the spark jump from the port on the cap to the end of the wire if it isn't far enough away, so it looks like both cylinders are getting spark.

But still...cylinders 3 and 5 produce no discernible change in the idle when I pull the wires off.

I re-re-re-verified the timing. Still right on at 10 degrees BTDC.

What's next? Am I looking at an injector problem or something?

Injectors. Look at the TEWD and see if those two are paired to one specific line... the injectors are wired in pairs, but I didn't think 3 and 5 were paired to each other.



EDIT:
3 and 5 are paired together, MK3 TEWD On-Line

Hmmmmm...so it appears maybe to be a potential wiring problem then. I'll have to take the sheath off and follow the wires back.

Other than that...what...ECU?

Yup. Test the signal wire, then ground wire, then ECU.