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1G What all can cause a misfire?

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GSLENK

10+ Year Contributor
1,416
51
May 25, 2011
DC, Maryland
I know, not another misfire thread... But all my searches pretty much bring up the usual suspects, or the 1g/2g swap which is already well documented by now. Also, the bench tests for the ignition parts I feel are not all guaranteed to confirm good parts.

I am on a little bit of a budget here. I am not looking to spend ~$500 on coils and/or power TR unless absolutely necessary to guarantee ZERO misfires. I did find the testing literature for both and they "passed" as of last year (but we know what that means).

Quick and dirty on the relevant mods (pushing ~60lb/min, but not much when the miss occurs):
Ignition:
Factory. NGK wires, NGK BR7ES plugs. OEM and original everything else.
Fuel:
AEM 320 E85 pump
-6an PTFE SS feed and return lines
Fuel lab regulator at 43psi
Fuel lab SS mesh filter 40 micron I think
FIC BM1250cc
OEM fuel rail
Run E85 mostly, 93oct at least once every 2 months.
Air/Timing:
ECMlink v3 Full on Speed density
Omni 4-bar/GM IAT
Run both LC1 WB AND NB OEM sensor

So lets get a list of all the possible causes of a misfire on a 1g/6-bolt.
I know for sure:
--Plugs
--Wires
--Power TR
--Coils
--Injectors
--Fuel delivery
--Boost/vac leak

Is there anything else? (please confirm OR DENY these) (has any of these been known to cause a misfire as described, particularly an INTERMITTENT/seemingly random one)?
--Compression?
--CAS?
--TPS?
--Leakdown test (HG failure)?
--ECU?

Here's my trouble shooting so far:

--Plugs (BR7ES): At .025 replaced because they are cheap. Did nothing. See attached pics if there are any clues on plug condition after a year of use. BPR6ES and/or high gap has seen blowout on my FP red at 33+psi so the current ones may foul, but they treat me well otherwise, and I replace them relatively frequently (at least yearly).

--Wires: One year old. Maybe 2 years. NGK blue wires. Did not see any arcing when I check on a recent night. I did not check that thoroughly, but they "seem" ok.

--Power TR: Replaced by a used one from classifieds. Both the old and "new to me" passed a bench test. Seller claimed his was flawless. His feedback was good so I no reason to think otherwise.

--Coils: Never touched them. I did bench test them but they checked out ok, and I believe bad ones can still test ok.

--Injectors: "New but installed" so technically very light use. Seller feedback, receipt, smell, and seal condition support his claim that they did not see any significant use, if any at all. They pass the stethoscope test. They all work. Pulling the electrical clip makes the engine really misfire for each one.

--TPS has .63v at closed throttle. And the throttle switch works. ISC is adjusted perfect too.

--CAS was removed, cleaned, installed backward, then reinstalled correctly when I did valve stem seals. Car ran beautifully for about 2 days, then back to normal. (hence me asking about the CAS).

--Mechanical timing was also verified at that time.

--Base timing was verified next (base ignition advance 5BTDC).

--Compression was checked 2 years ago and was excellent. (high and consistent). I have no reason to suspect otherwise to this day, but I have a tester.

--I have talked about swapping ECU's but no one has taken me up on the offer of a nice 6 pack to borrow their ECU for some driving time.

--Last season I really cracked down on boost leaks (no system is perfect, but I was pretty damn close). I have not touched any of those parts of the car to warrant another check. Besides, I run SD.

Nothing else has been physically replaced or tested.

Anything else I should do or redo?

Onto the symptoms:

I can hear a random miss every now and then (every couple of seconds) no matter the temperature of the car. Rough driving seems to help cause really bad misfires, especially if the car is warm/hot or specifically on a hot day. Rough as in jerking the clutch, low rpm (~1-2k) driving, really sharp corner steering, or tough bumps. Combinations appear to make it worse. Sometimes I need to rev to 3k to get the car to move under its own power, but this is really rare, and seems to only happen when it is really hot or really rough driving or both.
Hot outside, and in the bay. My engine temps seldom go over 210 if ever, and typically stay in the 186-196 range. Up to 203 on a hot day. Turning the AC on drops me to 193-196 any day even on cold/full blast. Keeping the engine at 196 does seem to help but that's all I can say.

Car is in a pretty good state of tune. Stacey did pretty well, the misfire was not really detectable while he was tuning. It began to really rear its head recently. (missing one ignition event out of 20 or so is not all that significant, especially at full throttle). Missing a few in close proximity brings the jerks/boost/power loss. The mis clear up above 4k easy, but I can feel the lacking power, even if the car feels smooth.

On a misfire event, the AFRatioEst will spike lean. On the really bad event it will rapidly cycle between target and max lean (20:1) about as quickly as the car sputters. Measured AFR may spike a little, but not as badly. Also when the car is "moody" AFRatioEst tends to be estimated way leaner than mapped, and way leaner than the actual afr. This is not typical when the car is running relatively smooth, the two flip flop etc. and are relatively close.

I have tried to use a timing light on each plug wire, I could not draw any solid conclusions it looked like the misfire might jump around, but idle was not steady enough throughout the misses to determine where it was coming from. When a big miss came up, the engine speed dropped so I could not tell if cylinders were dropping or it was an artifact of reduced rpm.

ANY OTHER INFO NEEDED?

I am really tempted to just buy all new OEM coils, TR and wires but that's quite a hit and if they don't make it work I will be mad and broke.

Also, I can get Coils and a Power TR on ebay NEW for really cheap. They do not appear to be OEM, but I have see the inside of a TR, it looks simple enough to warrant aftermarket. Coils are another story, windings must be good so I can understand that. Transistors and resistors are pretty hard to screw up these days (and are really cheap).

Any thoughts on knockoff Ignition components? They are REALLY tempting right now.
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You provided a ton of info, but back to simple. While the engine is running squirt water from something like an old simple green bottle on the ignition wires and coil. Do it a piece at a time to isolate the issue. In the dark look for archs or listen for the miss. If no change swap the injectors with a known good set and monitor results. Being inconsistent a mechanical issue is unlikely.
 
I should be looking for water to induce the problem? I'm on it. Im guessing, do not spray the CAS directly? Only spray wires and coils? Does it have to be dark outside?

BTW, turns out the Power TR Ignition Module is likely NOT the culprit. I bought a new OEM one to cast out any shadow of a doubt. And so the money pit journey begins... I have this feeling in the back of my mind that it could be the CAS, but all my searches for a CAS causing a random misfire are on 2g 6bolt swaps. I have never read about one on a normal 6-bolt.

Probably gonna get plugs and wires before the day is over. That would leave the CAS and Coils.

EDIT:
No results on spraying water. Ill wait till it gets dark.

It felt like I dropped multiple cylinders at one time when I did a test run after swapping to the new power TR. Of course this happens only after I run the car extra hard and hot.
 
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Well the misfire is back. New ptu and coil didn't do it. I found mine in eBay new old stock. Mitsubishi labeled and everything. Looking at other options. What's left?

Compression/leak down
Boost leak (doubtful, but you never know)
CAS? Could that be a cause?
 
I bought an oem ptu at oreillys, and an oem coil pack from rock auto a couple months ago.

I've had mixed luck with Rockauto parts. I just thought maybe there was a source on real McCoy Mitsubishi coil packs. Unsure if the the coils are even my problem.

Well the misfire is back. New ptu and coil didn't do it. I found mine in eBay new old stock. Mitsubishi labeled and everything. Looking at other options. What's left?

Have you double checked the injectors? As in the wiring to the little harnesses and the harness plug's prongs. Those can get soaked in oil and grease, they go through multiple heat cycles, and just deteriorate over time.

Also, the rubber O shaped rings that seal between the pintle caps and the head itself. Those have been a source of leaks on every DSM I've owned so far.

Injectors are where I'm leaning towards in regards to my misfire problem. I have a similar setup to yours with ECMlink and speed density. But even when I do a boost leak test, the injectors are not a source of leaks. However, when the motor's up to operating temperature and things are hot, I can wiggle the injectors and misfires come and go.
 
The coil pack from rock auto was oem, as well as the ptu from oreillys. With the age of these cars parts suppliers buy up oem supplies getting dumped out of the warehouses. They work fine at 40psi.
I'm not sure what your issue is.
I've had mixed luck with Rockauto parts. I just thought maybe there was a source on real McCoy Mitsubishi coil packs. Unsure if the the coils are even my problem.



Have you double checked the injectors? As in the wiring to the little harnesses and the harness plug's prongs. Those can get soaked in oil and grease, they go through multiple heat cycles, and just deteriorate over time.

Also, the rubber O shaped rings that seal between the pintle caps and the head itself. Those have been a source of leaks on every DSM I've owned so far.

Injectors are where I'm leaning towards in regards to my misfire problem. I have a similar setup to yours with ECMlink and speed density. But even when I do a boost leak test, the injectors are not a source of leaks. However, when the motor's up to operating temperature and things are hot, I can wiggle the injectors and misfires come and go.
 
Did you try replacing the noise capacitor (very cheap) on the coil's primary circuit? It's usually a small tin can bolted to the intake manifold with 1 wire coming out of it. Or even just try disconnecting it to see effect (easy test and engine should still run). Just in case it's partially shorting at times.
 
Did you try replacing the noise capacitor (very cheap) on the coil's primary circuit? It's usually a small tin can bolted to the intake manifold with 1 wire coming out of it. Or even just try disconnecting it to see effect (easy test and engine should still run). Just in case it's partially shorting at times.
So if it is working correctly, disconnecting it will degrade engine running performance and cause misfires?


Have you double checked the injectors? As in the wiring to the little harnesses and the harness plug's prongs. Those can get soaked in oil and grease, they go through multiple heat cycles, and just deteriorate over time.

Also, the rubber O shaped rings that seal between the pintle caps and the head itself. Those have been a source of leaks on every DSM I've owned so far.

Injectors are where I'm leaning towards in regards to my misfire problem. I have a similar setup to yours with ECMlink and speed density. But even when I do a boost leak test, the injectors are not a source of leaks. However, when the motor's up to operating temperature and things are hot, I can wiggle the injectors and misfires come and go.

Thanks for the heads up. I will investigate. I had replaced all the injector seals last year with a new set from FIC. Looks like next weekend a big time health check is in order. Last time I checked injectors, there was a little black e85 goo. I do run 93oct every month or every other month.

Plugs are gapped down to .020-.022 (but all close to each other).
 
So if it is working correctly, disconnecting it will degrade engine running performance and cause misfires?
No. The capacitor is to suppress electrical noise which can be heard through the radio. It does not affect engine performance unless it is bad (example: if it has an internal short at a voltage lower than it's rating in which case it would affect ignition). So temperarily disconnecting it should do nothing. But if all the sudden the engine runs properly when disconnected, you've found the problem. It's rare but easy to test.
 
Especially if you still have the green top Cas the black top style from the 93-94 are the best if you can find one
What does a CAS failure feel like?

My car is mostly derivable and smooth, but every now and then it goes nuts, and I lose all part/low throttle drive-ability. Drives smooth from a relatively cold start almost always, then gets hot and goes crazy sometimes. Engine temps stay normal according to link mostly around 196-200. Maybe a 206 spike every now and then.

My misfires come in spurts and are temperamental. Sometimes its really bad, other times the engine purrs smoothly. Most of the time I get what feels/hears like a random miss.

Unplugging injectors one at a time makes the car worse each time, it kind of masks the misfire when the engine is misfiring, but it is not like I pull an injector and nothing different happens.

Is CAS heat sensitive? It seems like the engine is more tempermental while things are really hot. But even on startup on a bad day, I can feel misfires. I can hear them in the exhaust, and can feel them in what comes out of the tailpipe.

I had a brand new OEM NOS CAS green top lined up on eBay for $100 + shipping. Worth it?

Is the black top CAS plug and play? I actually ordered 5x OEM orings for the CAS thinking that was my culprit for no reason other than desperation 2 months ago.
 
I was just thinking, wouldn't a bad CAS show up in a log? or can I have a "good" RPM signal and still get misfires etc... caused by the CAS? Not trying to knock the help. Just trying to cinvince myself to pony up the $50 for a used one or the $100+ for a new one...
 
Where are you at in Maryland? You're welcome to swing by and swap in some stuff from my parts hoard. I have pretty much every ignition part there is, and I can verify it's known good. We could even swap in the COP from my Galant if you want.

How about fuel? Have you ever pulled the pump and replaced the pump sock? The fuel tank clean? Fuel filter changed recently?
 
I'm in silver spring MD. I'd be willing to drive over. At this point I'm a little suspicious of the injectors since I do run E85. I had checked all of the fuel components last summer. I run all E85 compatible parts. Ill check all of that stuff, tank to rail for goo or other contamination before I decide to swing by. The miss seems too random and not consistent enough to call it an injector. I will also check compression/leak down too. Maybe I just have dirty valves from when I had a bad valve stem seal oil leak. It would be nice to verify that it is not an ignition problem.
 
You know what.....it could be the engine wiring harness its self. The electrical components can all be in good working order but they might not be receiving the signal due to a break in the wiring. It makes sense, heat and vibration related problems screams wiring in my experience. I seen it all the time when I was a heavy duty tech working on semi's and trailers. Had to rewire more trailers and trucks then you could shake a stick at because of problems just like this.
You've replaced the transistor, coil pack, plug, spark plug wires, and double checked your injectors, whats left??
The hardest thing would be to nail down the problem circuit. Its worth a shot.
 
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New CAS did not do it... I hate throwing money at this thing. Also, I get a really bad break up or misfire when I go 100% Sucks. New plugs and gapped down to .020 Seems tight enough... Go tighter?
Oh how I know the feeling.... Another thread, about the latest pandemic sweeping across the DSM community.
The strangest thing is swapping out parts doesn't remedy anything.....
Hey I'm sorry to regurgitate comments here, but check your ECU. If you can plug your BOV or replace the intercooler pipe with no BOV try that. Pull the VC of check for fallen rockers.. Do you have a WB? Did it go lean or rich.? If rich ignore your fuel, if lean check pump injectors and regulator
Me and many others are rooting for you, cause we are all in the dark to right now.
 
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