Trying to diagnose random misfire.

[yotatake2]

Hi Guys, I'm at my wits end here and I need some help. I've been trying to diagnose a random misfire on my truck (99 3.4 4x4 with 250k miles) for the past week now. I'm going to try and document what I've done so far in as organized a fashion as possible so here goes.

Problems:
- For the last few months, if the engine is at a low rpm, it will sound very gluggy (i know, very descriptive) almost as if it's chugging. (still persistent)

- For the last few months, I've been getting lower than usual mpg. I know driving aggressively and having a full sized spare on my roof rack probably don't help, but 13mpg?? really?? (still persistent)

- For the last few months, if I'm cruising slowly in traffic (10-15 mph) and I lightly blip the throttle, the truck will bog, then rev up. I switched to 89 octane and this problem went away. But I'd like to go back to 87 if I can fix the underlying issue (temporarily fixed)

- For the last few months, when I'm accelerating, if I abruptly take my foot off the gas pedal, the truck will stumble very hard. It mainly happens when it's under a good amount of load (3-4k rpm) and I suddenly close the throttle. (still persistent)

- For the last couple of weeks, when I cold start the car, it will run very rough and misfire for a few seconds then, if I either blip the throttle or give it a few more seconds, it will run fine. (still misfiring, but constantly now)

- Last week, I tried to drive the truck to my friends house and it sounded like it wasn't running on all cylinders. There was also a major lack of power. (fixed with new coil packs)

- Currently, it has a random misfire but isn't throwing any codes

Trouble codes:
P0300 gone
P0100 gone
P0110 gone
P0120 gone

What I've tested/replaced and problems they've fixed:

Replaced (what it fixed):
fuel pump (nothing)
coil packs (now running on all cylinders, still randomly misfiring)
spark plugs (nothing)
spark plug wires (nothing)
TPS (nothing)
MAF sensor (nothing)

Checked (and confirmed to be good):
Crank position sensor
Cam position sensor
Compression
Cleaned the MAF (visually looked okay)
Vacuum leaks (there's a few I know of that the truck has, at one point, run fine with, but I checked for any other ones and couldn't find any with starting fluid.)

Suspects:
MAF?
Ignition Control Module?
ECM?
Vacuum leaks (one small, open copper tube at top of throttle body that has been open since I got the truck and one hose that used to go to the intake silencer before I deleted it. But it ran fine with both of these leaks for a while)

I'm going to be getting a new MAF today and will try swapping that out. Any other suggestions? Does it sound like it could likely be the MAF?

Thanks guys! I will keep researching in the mean time.

 

[Rock_Rat]

Hi Guys, I'm at my wits end here and I need some help. I've been trying to diagnose a random misfire on my truck (99 3.4 4x4 with 250k miles) for the past week now. I'm going to try and document what I've done so far for you in as organized a fashion as possible so here goes.

Problems:
- For the last few months, if the engine is at a low rpm, it will sound very gluggy (i know, very descriptive) almost as if it's chugging. (still persistent)

- For the last few months, I've been getting lower than usual mpg. I know driving aggressively and having a full sized spare on my roof rack probably don't help, but 13mpg?? really?? (still persistent)

- For the last few months, if I'm cruising slowly in traffic (10-15 mph) and I lightly blip the throttle, the truck will bog, then rev up. I switched to 89 octane and this problem went away. But I'd like to go back to 87 if I can fix the underlying issue (temporarily fixed)

- For the last few months, when I'm accelerating, if I abruptly take my foot off the gas pedal, the truck will stumble very hard. It mainly happens when it's under a good amount of load (3-4k rpm) and I suddenly close the throttle. (still persistent)

- For the last couple of weeks, when I cold start the car, it will run very rough and misfire for a few seconds then, if I either blip the throttle or give it a few more seconds, it will run fine. (still misfiring, but constantly now)

- Last week, I tried to drive the truck to my friends house and it sounded like it wasn't running on all cylinders. There was also a major lack of power. (fixed with new coil packs)

Trouble codes:
P0300 gone
P0100 currently
P0110 currently
P0120 currently

What I've tested/replaced and problems they've fixed:

Replaced (what it fixed):
fuel pump (nothing)
coil packs (now running on all cylinders, still randomly misfiring)
spark plugs (nothing)
spark plug wires (nothing)
TPS (nothing)

Checked (and confirmed to be good):
Crank position sensor
Cam position sensor
Compression
Cleaned the MAF (visually looked okay)
Vacuum leaks (there's a few I know of that the truck has, at one point, run fine with, but I checked for any other ones and couldn't find any with starting fluid.)

Suspects:
MAF?
Ignition Control Module?
ECM?
Vacuum leaks (one small, open copper tube at top of throttle body that has been open since I got the truck and one hose that used to go to the intake silencer before I deleted it. But it ran fine with both of these leaks for a while)

I'm going to be getting a new MAF today and will try swapping that out. Any other suggestions? Does it sound like it could likely be the MAF?

Thanks guys! I will keep researching in the mean time.

Replace or clean the MAF and check out the connector and wires. Make sure they're not damaged. When was the last time you cleaned your air filter? Are you running a K&N or oil type filter? Might need cleaning. The IAT is inside the MAF ( It looks like a Q-tip dipped in oil) and those codes are directly related to the IAT except the P0120 which is your TPS. That code is probably being tripped because of the MAF.

Here's a link to my post on the MAF. Hey don't STRIP the small screws!!

EDIT: ONLY USE OEM for the MAF!!!!! You want the highest quality PCB you can get. Your computer relies on your MAF!!! I ordered mine from Rock Auto.

 

[yotatake2]

When was the last time you cleaned your air filter? Are you running a K&N or oil type filter? Might need cleaning.

I recently changed the air filter with a paper one. I forget which brand at the moment.

Hey don't STRIP the small screws!!

Too late lol. I'm going to find some replacement screws and drill out the stripped one.

EDIT: ONLY USE OEM for the MAF!!!!! You want the highest quality PCB you can get. Your computer relies on your MAF!!! I ordered mine from Rock Auto.

I'm thinking of getting a Duralast one for now just to see if that's the issue and if it is, I'll spring for the OEM one.

 

[LittleCaesar]

OEM for the MAF only, or I were in a pinch possibly would use Denso. NOTHING else.

Check the wiring to the MAF and IAT for opens. It get brittle and if it gets twisted enough it can break. Also check all body grounds (make sure they are nice and tight).

 

[yotatake2]

Hey guys, so last night I replaced the MAF and cleaned my original MAF and it still misfired with both in there. I replaced my spark plug wires because I noticed the old coil packs I had in there had alot of corrosion at the contact points and still no luck. The car will misfire fairly regularly on idle and under load. Where do I go from here. My friend who's been helping me this past week suggested I take it to the stealership and tell them exactly what I've tested and replaced and ask them to check the ICM and ECM with their diagnostics tool.
Any other ideas before I dish out the $100 to get it diagnosed?

 

[kolelt]

Could a bad ECU be causing this ?

 

[yotatake2]

Could a bad ECU be causing this ?

I really really really really really really hope not lol. Those suckers aren't cheap. And how often do they really go bad?

 

[trovergraywilson]

I'll throw out the I'm-not-a-mechanic card, but this sounds familiar.

A couple of years ago, my partner got his dream car - 2007 Audi A4, cabrio, 2.0L turbo four. Thing was beautiful sitting still, but if you so much as cranked the engine the problems started. Misfires, shuddering, bogging, power interruption, burning oil, etc. We tried everything. Ended up being a fuel injector that had gone South and was vacillating between being completely shut off or stuck open. New injectors all around fixed the problem and we traded it the next day for a Lexus (Toyota FTW!) and haven't looked back. Never again, Audi. Hope this helps.

 

[yotatake2]

I'll throw out the I'm-not-a-mechanic card, but this sounds familiar.

A couple of years ago, my partner got his dream car - 2007 Audi A4, cabrio, 2.0L turbo four. Thing was beautiful sitting still, but if you so much as cranked the engine the problems started. Misfires, shuddering, bogging, power interruption, burning oil, etc. We tried everything. Ended up being a fuel injector that had gone South and was vacillating between being completely shut off or stuck open. New injectors all around fixed the problem and we traded it the next day for a Lexus (Toyota FTW!) and haven't looked back. Never again, Audi. Hope this helps.

Glad you joined the Toyota family! My friend did also say it could be fuel injectors. I've run the tank empty a couple times because I'm a dumbass and it wouldn't be immpossible for a tiiiiny bit of sediment to get past the fuel pump strainer and fuel filter and lodge into an injector. Or it could just be a sticky solenoid in an injector from age. I will look into those.

 

[LittleCaesar]

Losing any coolant?

 

[yotatake2]

Losing any coolant?

not a substantial amount, but I just realized my cap is leaking so it could very well be from there. You thinking head gasket? I did a compression test on all 6 cylinders and they were all in spec. Actually, the back 2 were 210 psi, so waaay above spec. The rest ranged from 165 to 190. The variance does worry me though.

 

[Rock_Rat]

If it were me, I would do the following in this order:

• Double check the routing of my wiring just to make sure I have everything routed correctly.
• Look at all vacuum lines and check for leaks beginning at the manifold (starting fluid)
• Remove the plugs and examine closely to ensure they're firing. Check the gap again. (Denso or NGK)
• Look in each cylinder and see if one is more wet than the other
• Do a resistance check on the coil packs (I know they're new.)
• Once all is good, reinstall and ensure wires going into the coil packs are correct
• Finally I would do a voltage check on the fuel injector connectors. Passenger side is easy (1,3,5, but I think the plenum needs to come off for the drivers side 2,4,6)

I cannot emphasize enough how important it is to use OEM sensors or Denso. I know this first hand with knock sensors.

Until you can completely eliminate the spark and fuel delivery system as being 100% OK, you're only going to be second guessing.

I would say with a 99% certainty that there's nothing wrong with the ECM. It's doing its job and providing you with codes to help isolate the malfunction.

Be patient and take it one step at a time.

EDIT: As an FYI, most software vendors build in a timer delay for codes to latch. Sometimes it's >10 seconds. this depends on the severity and level of the code. If they didn't do this we would get these spurious codes anytime there's a hiccup in the system. I say this because you don't mention any injector codes. I'm not sure what it takes to latch the injector codes. You still can't rule out injectors.

 

[LittleCaesar]

If there is any coolant being lost then buy a new cap, OEM if possible, and also check to make sure the cooling system is holding pressure by renting one of these from an auto parts store. I believe the 4runner is at 15psi:
https://www.amazon.com/gp/product/B...d_t=201&pf_rd_p=1944687642&pf_rd_i=B003V9L05G

Make sure you are performing the compression check on a hot engine, throttle wide open. The variance between the numbers you posted is too great, it should not be more than 10%.

 

[jkoni426]

Seems like you've replaced/checked everything but fuel injectors. Maybe dump a can of seafoam in your gas tank with your next fill up and see if that helps. If not, try testing them with a multimeter before just replacing them. Better to test first than to throw money at a problem hoping it fixes it.

 

[kolelt]

I really really really really really really hope not lol. Those suckers aren't cheap. And how often do they really go bad?

Nah they cheap actually. eBay has plenty. But that could be ur last chance

 

[yotatake2]

I'm thinking of getting my injectors rebuilt and tested. But some people were recommending I do a leak down test. Given the variation in my compression readings, would you guys recommend that?

 

[yotatake2]

I talked to a friend who's a toyota tech who knows these engines well. He said to check the valve lash. If it's too tight, it could explain why the compression was so high in the back and it could cause a misfire. I checked the valve lash and they were all within spec, so now I'm planning on sending the injectors out for cleaning since they're easily accessible now. Then I'll put everything back together, and redo the compression test just to double check. If all that doesn't fix it, I'll do a leak down test. Then, if that turns out good and the misfire is still there, then I'll officially be out of ideas lol.