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?