1988 7MGE Misfire - Stumped!

[cre]

Originally Posted by excaliburdj

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.

Good, so that's out.

Originally Posted by excaliburdj

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

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

Originally Posted by excaliburdj

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

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

[excaliburdj]

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?

[cre]

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.

[excaliburdj]

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?

[cre]

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.

[excaliburdj]

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

[cre]

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

[cre]

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.