Misfire after replacing Spark & Injectors

[prybar]

Hi all,

Been dealing with a real irritating misfire and high fuel trims.

To begin, I've been seeing around -10% LTFT and STFT fluctuates around +-5%. This is from Torque Pro. A few months ago, I swapped out both O2 sensors along with my MAF sensor - all Denso. Disconnected my battery to reset everything, and fuel trims appeared fixed at first, but reverted back to -10% within a day or two.

I got a misfire last week while driving on the freeway. No big deal, I ended up swapping out the spark plugs (yes, I gapped them), wires, & coil packs. The truck ran marginally better, but the misfire still occurred.

Next thing I suspected was injectors. Didn't have the time to do it myself, so I took it to a very reputable Toyota shop in my area (Red Sun in Rocklin, CA - would recommend). They confirmed that two of the injectors were bad, so I just had them replace all 6 with Densos. Got the truck back and it ran fine, no misfires right away.

The first thing I noticed was that my fuel trims are now higher negative. Up to -14% LTFT and STFT fluctuates from +5% to -10%. While driving at speed, STFT is closer to 0%, but when I let off the gas while driving or after coming to a stop, STFT can jump up to -10% (in conjunction with the -10% LTFT). I was being hopeful and thought maybe the ECU was just relearning itself.

Yesterday I went for a long drive to test everything out. It felt fine, but the fuel trims were heavy on my mind, I've never seen them that high. I drove up a hill at 50 mph, and coasted down the other side, still around 50 mph. When I came to a stop after coasting, I got a single misfire. It only lasted a second but was enough to tell me that a problem was still there.

My thinking on what to try next: fuel pressure regulator or EVAP purge valve. My question for you guys is am I on the right track? How can I test if these two things are failing? Is there anything else I should look into? I'm not getting any codes at all and am starting to run myself ragged after replacing all these things. Any help would be greatly appreciated.

 

[Ben Dejo]

I have a similar issue with my 99 thats just about to roll over to 300k. I've changed the injectors to the ones from Motorwest when I had a injector failure, then the O2 sensors, MAF, coils, wires, plugs, all the vacuum lines replaced. Parts cannon wastes money is all I have learned. I dont get misfire codes since I replaced the injectors, but on occasion I feel the engine shutter like a misfire. From what I have read in these forums, fuel pressure regulator failure manifest differently to what we have going on. Not sure what issues the purge valve would cause but I expect it would throw a code for EVAP. I havent checked the actual fuel pressure yet to check for a fuel pump issue. On occasion when at idle the STFT goes to -15, which is cutting fuel and it stalls itself out because it cut too much fuel. In the last 14 months, I get what I assume is pinging at low RPM but only under load, like going up steep incline. Got me on why, it isnt fuel grade, 87 oct is 87 oct and its not done this for the +20 years Ive had the 4runner. One of these days I will get the boroscope out and look for carbon buildup in the cylinders and probably check the cat via the front O2 sensor. Doubt I will find out why its doing what it does. I still get 18-20MPG in the city. If you find a solution, I would be interested to know.

 

[98Gate]

I have a similar issue with my 99 thats just about to roll over to 300k. I've changed the injectors to the ones from Motorwest when I had a injector failure, then the O2 sensors, MAF, coils, wires, plugs, all the vacuum lines replaced. Parts cannon wastes money is all I have learned. I dont get misfire codes since I replaced the injectors, but on occasion I feel the engine shutter like a misfire. From what I have read in these forums, fuel pressure regulator failure manifest differently to what we have going on. Not sure what issues the purge valve would cause but I expect it would throw a code for EVAP. I havent checked the actual fuel pressure yet to check for a fuel pump issue. On occasion when at idle the STFT goes to -15, which is cutting fuel and it stalls itself out because it cut too much fuel. In the last 14 months, I get what I assume is pinging at low RPM but only under load, like going up steep incline. Got me on why, it isnt fuel grade, 87 oct is 87 oct and its not done this for the +20 years Ive had the 4runner. One of these days I will get the boroscope out and look for carbon buildup in the cylinders and probably check the cat via the front O2 sensor. Doubt I will find out why its doing what it does. I still get 18-20MPG in the city. If you find a solution, I would be interested to know.

Gosh, something in the water right now. I am having the exact same issues as you. Have the motorwest injectors as well. No code, no misfires but terrible fuel trims. The last thing on the list is the fuel pump….. i I dont see how the pump could make the truck run rich though. Fuel pressure regulator is new as well, I dont think that could do it either. My ideas are valve clearance issues, may be worth doing a leak down. Maybe, just maybe MAF is performing correctly at idle but under a load is reading incorrectly. I am stumped. I might take one for the team and rebuild the old injectors I took out, put new O rings on and tell yall what happens. There has to be a diagnostic procedure for this….,

 

[prybar]

Gosh, something in the water right now. I am having the exact same issues as you. Have the motorwest injectors as well. No code, no misfires but terrible fuel trims. The last thing on the list is the fuel pump….. i I dont see how the pump could make the truck run rich though. Fuel pressure regulator is new as well, I dont think that could do it either. My ideas are valve clearance issues, may be worth doing a leak down. Maybe, just maybe MAF is performing correctly at idle but under a load is reading incorrectly. I am stumped. I might take one for the team and rebuild the old injectors I took out, put new O rings on and tell yall what happens. There has to be a diagnostic procedure for this….,

Yeah, something in the water. I ended up taking it back to the shop that swapped my injectors, and their lead mechanic told me he couldn't find anything wrong. He said he didn't change anything, but my fuel trims are much better now after he tinkered with it. Now I'm seeing up to -10% LTFT at idle while STFT sits near zero, when before they would be both be pegged at -15%, causing a misfire.

My thinking is that I'm not actually running rich, and the misfire comes from the ECM pulling too much fuel (ie running lean). Still running rough at idle, but no misfires since I got it back from the shop the second time. If you find out what the issue is, let me know! I'm just gonna run with it for now I guess, until the issue becomes more apparent. I checked all my sensors (MAF, O2 sensors, etc) and they all came out good. Even tested with a known-working MAF sensor and have the same results.

Now that it's only running rough at idle, gonna tear into my throttle body this weekend and clean it out good. I already did this a couple weeks ago but it was over 100* out and I was in a rush so i could have missed something.

 

[98Gate]

Yeah, something in the water. I ended up taking it back to the shop that swapped my injectors, and their lead mechanic told me he couldn't find anything wrong. He said he didn't change anything, but my fuel trims are much better now after he tinkered with it. Now I'm seeing up to -10% LTFT at idle while STFT sits near zero, when before they would be both be pegged at -15%, causing a misfire.

My thinking is that I'm not actually running rich, and the misfire comes from the ECM pulling too much fuel (ie running lean). Still running rough at idle, but no misfires since I got it back from the shop the second time. If you find out what the issue is, let me know! I'm just gonna run with it for now I guess, until the issue becomes more apparent. I checked all my sensors (MAF, O2 sensors, etc) and they all came out good. Even tested with a known-working MAF sensor and have the same results.

Now that it's only running rough at idle, gonna tear into my throttle body this weekend and clean it out good. I already did this a couple weeks ago but it was over 100* out and I was in a rush so i could have missed something.

I would assume cleaning the IAC and be sure to order a gasket from rockauto(along with any other random stuff you need to make the shipping have some bang for your buck). Maybe a set of brake pads or something you know you’ll need in the future? Getchu a toothbrush, throttle body spray and get to work on it. Might be worth it to use electrical parts cleaner on the IAC itself since it does have some sensitive electrical components on the side of it. Probably wont matter if you use the throttle body cleaner on that too tho. You and I both are up there in milage now so the motor is probably just not as tight as it once was. Needless to say, it always gets me where I need to go. sounds like you’ll have that thing purring at idle in no time!

-98Gate