It's been a while since I've posted anything... good to be back! I've been fighting an engine misfire that's eluding me. I'm hoping someone here may have some suggestions.

To start with, my 1999 4Runner (V6 / Limited) has 193k miles on her.

A few months back I performed the following list of maintenance. This was primarily in response to both valve covers suffering from a very minor leak. Since I was getting my hands dirty, I figured I'd take care of various other items I knew needed to be done. All replacement parts are OEM.
-Replace both valve cover gaskets
-Replace all spark plugs (gapped them per spec)
-Replace all spark plug seals (in the valve cover)
-Replace fuel filter
-Replace front O2 sensor
-Oil/filter change
-Radiator flush, refilled w/ Toyota red.
-Cleaned MAF
-Cleaned throttle body & IAC valve

With that out of the way, the truck ran great for a month or so. However, the truck began misfiring with a fairly common pattern. I'd drive the truck around for a while, park it for a bit (15 - 60 minutes), and when it fired back up, it would run rough, both at idle and at driving speed. Letting the truck sit overnight would resolve the issue. OBDII showed a P0302, and eventually a P0171 would follow.

Easy enough, there's plenty of info out there about this particular issue. I started by moving my spark plugs around to ensure the new ones weren't defective. No change. Replaced my spark plug wires. No change. Moved the coil pack and wires around to see if the issue would follow the parts. No change. Swapped out all six fuel injectors. No change. Moved the new fuel injectors around to different spots to verify they were all good. No change. Oddly enough, for the first day or two after making a change, the truck would drive fine, but the issue would always return within a week.

During this whole ordeal, the problem got worse. I started seeing P0301 shortly after P0302 (but it always starts with P0302 and only sometimes expands to P0301). As expected, P0300 shows up when both cylinders are misfiring. The problem seems to be exacerbated when the outside temperature was abnormally hot. Cooler temperatures mitigate the issue somewhat.

EDIT: I forgot to mention that the misfire isn't always consistent. It can be misfiring for 5 minutes, then out of the blue start running fine again. There's been many times where it bounces between misfires and "fine" a couple times per minute. I've tried finding a pattern to this behavior, but haven't had much luck. The misfire is most noticeable between idle and 2000 RPM; once I get above 2000 RPM, it appears to run much better, but I can't tell if the higher RPM is covering up the issue or actually resolve the issue.

On a side note, gas mileage does not seem to be affected much.

I'm at a bit of a loss. Any suggestions on what to try next?

 

How did the old #2 plug look? This would frequently give you a clue. Super clean = HG leak. Oil fouled equals ring problem, etc.

You might want to do a compression check, followed by a leak-down test.

 

The #2 plug looked clean. I compared it with the plugs in #1 and #4; they all look identical.

Compression test is on the to-do list. I appreciate the info!

 

I've had a crank sensor go bad and it took me a month to track down. Also had crank pulley bolt not get tightened down after timing belt replace and cause the pulley to walk a little, thus the crank sensor can't pick up properly.

 

Performed a compression test today. Cylinders look good from what I can tell. The results are at the bottom of the post for those who are interested.

After reviewing the FSM, one of the diagnostics steps is to check the voltage going from the ECM to the fuel injectors. Sure enough, the wire going to #2 showed no signs of life. All the other wires tested at 9 to 10 volts (9 to 14 is acceptable). When I installed the new injector, it had a resistance of about 14 or 15 ohms, so I presume it's good. To me, this means that either the ECM is failing (unlikely, as the ECM was replaced by the dealership just a few years ago), or the circuit running to the injector is intermittently failing.

My theory: since the problem only happens after the engine is warm and the vehicle has been driven a bit, and the wire runs across the top of the engine, the heat is exacerbating a short in the wire. I probably did some damage do it when I replaced the valve covers. This would also lend credence to the fact the problem will come and go while driving.

Does this theory sound plausible? Anyone have any suggestions on trying to track down the problem between the ECM and injector?

EDIT - For those of you looking at the FSM's ECM wire diagram for the fuel injectors (page EG-244), please note the pins to test are not correctly marked (at least not for my 99 4Runner Limited). The correct wiring diagram can be found here: http://www.mr2.com/files/mr2/techinf...Runner-ECU.pdf

Compress Test Results:
#1 - 145
#2 - 145
#3 - 140
#4 - 145
#5 - 150
#6 - 150

 

Woke up this morning and immediately fired up the 4Runner. She ran great. Tested all the ECM pins for the six fuel injectors. All were showing proper voltage. Drove the truck to work (30 minutes), parked for 9 hours, and drove home (40 minutes) without any issues. Fired her up about an hour after getting home to start working on this issue and she stumbled a bit; an obvious misfire.

So I tore the intake plenum off and checked all the wiring. No signs of damaged wires or shorts. In fact, the wires look surprisingly good for being 16 years old.

Tested the pins at the ECM again. All show proper voltage. Tested the injectors again. All show proper resistance. So I put a bit of Di-Electric Grease on the fuel injector connectors and put the engine pack together. Went for a quick drive and she ran great.

However, I let her sit for 10 minutes and fired her back up. There was a definite misfire. Checked the ECM pins again: #2 and #6 had no voltage. Turned the engine off but ignition in the ON position:#2 still has no voltage, but all other pins have voltage, including #6.

Looking at the wiring diagram I posted earlier, it looks like a black/red wire runs from the ignition fuse to all the injectors. Each injector then has a smaller gauge wire going to the ECM, which goes into an NPN transistor, and presumably to ground.

Given the physical composition of the engine, it's pretty much impossible to test the wires at the #2 injector while the engine is running.

I'm at a bit of a loss. Despite having a relatively new computer, I'm now starting to wonder if this may actually be an ECM issue. Ideas, anyone?

 

The ECUs are rock solid. Usually everyone who's had a computer replaced is just throwing parts at it, it's rarely/never the problem. Look elsewhere. Did you consider any information I posted above?

 

You might want to double check all electrical connections, and grounds. Make sure they are clean, and grounds are to bare metal.
How are the hoses, vacuum lines, I hear something about a vapor canister that can get clogged if you tend to top of when filling up.

 

Not sure if you have tested the coil packs?

Feels like a moisture/electrical issue to me.

 

@catusphile

Did you ever get this sorted out?
I also have a 300/301/302 combo, and it seems odd that 2 fuel injectors would fail at the same time.
Only misfires on a warm engine start.

Thanks!

- olaf

 

not so much. About 3 years ago there was a spate of folks that had misfires when warm. The common denominator was injectors. I had 2 go bad at the same time and received a DOA replacement that had “tested good”.

You can get an eBay set of injectors for around $100. It takes about an hour to pull the intake enough to get your hands in to change them out. The only real consumable is the gasket which was like $20 for a fel-pro here in MD.