Background: when it was working properly, on cold start it would go to 2000 rpm and when it warmed up it dropped to about 900. After low/barely any use for months (possibly years) it
started not reaching 2000 rpm and when fully warmed up it would stall.
Years ago, but only about 7000 miles ago, I neglected to replace the timing belt and it broke, taking some valves with it. Had a complete cylinder head rebuild job, new timing belt, new water pump. After that job it ran fine for quite a while. The current mileage is about 128K.

Past symptoms: at some point, it seemed like it was running really rich and the mileage was outrageously low, something like 5 mpg, but that symptom is gone. Possibly an injector was stuck open.

Replaced air filter, spark plugs, spark plug wires, distributor and rotor, but that didn't seem to make much of a difference.

Current situation: sometimes it stalls but it mostly goes to very low idle and has some misfiring, sometimes worse than other times. It seems that the worst symptoms have improved lately, not sure why.

The latest tests: with a timing light I noticed that cylinder 2 has very consistent spark, 4, 6, 1 and 3, very few misses, and 5 (longest wire) only has occasional spark per the timing light. Removed that spark plug (it's new and looks just fine), measured the resistance of the plug wire: 8.33 k Ohm, well within what the manual says: 8.5 for cylinder 2 (shortest wire) to 12 for 5 (longest wire). The manual says that the primary coil resistance should be between 0.72 and 0.88 Ohms. I measure 1.11. The secondary coil should be between 10.3 and 13.9 kOhms. I get 13.28.

Could the higher-than-spec resistance of the primary coil be the reason for weak spark all over? I'm tempted to just get a new coil but should this resistance difference be capable of such drastic effects? I've found this supposedly compatible coil:

https://www.rockauto.com/en/moreinfo...t=7060&jsn=116

But the primary coil resistance is 0.98 Ohms, also above what the manual says.

I intend to do a compression test as soon as I get my compression gauge back from someone that can't remember where he put it. But even if that cylinder had no compression, the spark should still be good and it isn't.

Any other suggestions?

Thanks.

 

Check your idle control solenoid. Low idle, stalling and misfures can all be coming g from that.

 

I presume you mean the solenoids of the idle speed controller. Per the manual the 6 prong connector should have the following resistance between various prongs:

2 - 1 and 3: 28-33 Ohm
5 - 6 and 4: 28-33 Ohm

My measurements are as follows:

2 - 1: 38.88 Ohm
2 - 3: 39.23 Ohm
5 - 6: 38.93 Ohm
5 - 4: 39.05 Ohm

A little over spec, but should this cause the problems I'm seeing? Years ago, I had to replace the idle speed controller because the engine would rev up then down then up, etc... One of the coils showed open (infinite resistance). A quick search has shown that the idle speed controller is about $400, so I'd rather not go buy one blindly if it's not needed. If anyone has a source that's reasonable I'd certainly like to hear about it.

Thanks.

 

Have you taken it out to examine it? It can spec OK on resistance, but may have some other issues - gunk buildup, ports clogged, other problems. My idle control solenoid failed in a mysterious way - the ring that holds the plunger came unglued, so the solenoid would randomly stuck closed or open. The computer would have no ability to keep idle correct because of that. The engine ran OK at mid to high RPM, but would nearly die or die on every turn and stop light when engine needed to be just at idle. The CEL code I had for it was Random Misfire...

 

I did previously. Everything looked normal except that it had a grey coating similar to what you'd see on a spark plug. The manual says not to use solvent to clean it. Wiping it does not remove this coating. Anything that is safe to do this? I sprayed it with carburetor cleaner but it didn't make any difference.

That is what I'm experiencing, but it's gotten a little better lately. I'm not able to read the computer codes for lack of the proper test tool. There were no obviously loose or broken parts that I could see. The manual has a procedure to test the operation of the ISC, which is also here:

https://mitsubi****echinfo.com/data/...3100530044.htm

The "operation check" refers to a test harness, part number MD998463, but does not specify what pins in the ISC connector the different leads are connected to. Does anyone have the detailed schematic of this test harness? The testing is quite rudimentary, after applying 6V, does the motor vibrate? if yes, "the stepper motor can be considered normal" This says nothing of how much (rotation angle or extension) is necessary for it to work properly.

I have recently bought an oscilloscope with signal generator, so I could generate a square wave and feed it to it and see what happens, but without knowing the full range of the plunger it wouldn't tell me much about proper operation.

More generally, can the ISC be disassembled and repaired? I have the old one somewhere, the one with the open coil. If that open coil could be replaced, I'd have a known good one.

My new scope (picoscope 2206B) apparently has a software module to decode automotive codes, so presumably I could, if I knew the electrical configuration of the computer port where the test tool is connected to the computer, read the codes. But as this would require creating a test harness compatible with the computer and the digital inputs of the scope, it's a problem for another day.

Incidentally, I tried a new coil and it didn't solve the problem. the new coil had a 1.1 Ohm primary coil resistance (higher than the original) and 11 something k Ohm secondary coil resistance (lower than the original and well within the manual's range (10.3-13.9).

Thanks for your help.

 

Idle Air Control Valve is $173 brand new at any auto parts store. This part virtually never goes bad. Using the generally reliable 10% rule you can get one of these for 20 bucks at any junkyard to eliminate it as a possibility...free if you bring them a dozen donuts as they know they will never sell it.

 

Actually, I had just ordered one on Ebay. Found it by part number (MD628053) and it only cost $24.50. Brand new Mitsubishi part. I learned that it is found in multiple models (3000GT, Diamante, Kia Sorento), probably those that use the same V6 engine. Still waiting for it but as soon as I get it and install it I'll let everyone know if it fixes the problem.

 

Well, the Idle Speed Controller didn't make any difference. It still idles way too low when hot, and adjusting the idle speed sensor doesn't make enough of a difference to make it work like it used to. It still feels like the spark is weak/marginal, as I described above but testing with a new coil didn't show any difference either. I have read somewhere that a vacuum leak could also be the problem. How could I test for this? I have a vacuum gauge somewhere. The hoses seem all to be in place and I don't hear any whistling.

 

Next test: the power transistor. The manual says that when 1.5 V is applied to terminals 1 (+) and 2 (-) there is "Continuity" between terminals 2 and 3. when no power is applied, there is "Non-continuity". I measure 6.88 K Ohms between terminals 2 and 3 and the same when 1.5 V is applied to 1 and 2. Tried many times and nothing changed. Then I inverted the polarity, this of course messed up the Ohm meter reading but then i saw a low resistance (around 3 Ohms) for a while. It looked like the inverted voltage had reset the transistor. I kept reading the low resistance for a while, but then it reverted to 6.88 K Ohm.

So, according to this test, since there is no difference between "Continuity" and "Non-continuity", I have a bad power transistor. But according to thie page:

https://easyautodiagnostics.com/mits...-tests?start=3

This test assumes that you have a 'no-spark no-start' condition. If your car starts and runs, then this is a clear indication that the power transistor is working fine.

So, do I have a bad power transistor? Am I correct in assuming that the power transistor provides a pulse to the coil, that in turn powers the spark plugs? Is the resistance of 6.88 K Ohm normal? If this resistance remains constant, could it make the pulse to the coil weak enough to cause the misfires I'm seeing? Or, as the web site above says, any malfunction in the power transistor would prevent the engine from running at all, and thus I can assume I have a good power transistor?

I can also not find the part number. Does anyone know the part number of the power transistor? a bit of searching has found a possible match: MD164475 which can be found here:

https://www.rockauto.com/en/catalog/...ule+(icm),7172

but it's listed for a 3000GT, although it probably uses the same engine. In any case, I'm not going to keep on buying parts if I'm not sure it's actually bad.

Can someone provide confirmation that the power transistor is either good or bad?

Thanks.

 

UPDATE: The misfiring part seems to have been solved, but at great cost: $1354.83. All of the sudden the engine stopped while driving and I could barely start it and at idle limp into a parking lot. As soon as I accelerated, there seemed to be no spark at all. I had to be towed to the local chain repair shop. After an $85 towing fee, a $50 "tow evaluation" (WTF?) and $100 diagnostic fee, they determined that the fuel pump had died. They replaced the fuel pump, the strainer, the fuel filter and the high pressure hose. Now the engine runs without misfires, but the idle is still low when hot. I'll have to go back and ask what error codes the diagnostic computer reported. It might help point to the cause of the low idle. I'm hoping that now, with good fuel flow, the computer will learn to idle properly eventually. We'll see. If not I'll keep working at this.