Misfire no code (1 Viewer)

[hawaiicruz]

Hi all,
It's been quite a while since I posted, been busy with a bunch of other projects and my 94 has been rock solid for quite sometime.
Anyway, I was driving for about an hour and I developed a shuddering misfire that gets worse under load. I limped it home and pulled a code 25. My intake hose was cracked and taped, so I assumed it was that , replaced hose with a good one from my 93 and drove it. misfire is still there but CEL is now off. So now I'm guessing o2 sensors or the whole "wiring harness next to the EGR thing". Any thoughts? Is it possible to have a misfire and not throw any codes?

 

[AaronM33]

How's your head gasket? Tune up stuff?

 

[hawaiicruz]

How's your head gasket? Tune up stuff?

Head gasket is solid. Tune up stuff probably overdue. It just seems strange that it's misfiring but not throwing any codes.

 

[dbug]

Maybe dirty injectors?
You could try Amazon.com: BG 44K Fuel System Cleaner: Automotive

 

[george_tlc]

Misfire (at least on obd2 models) is detected by the ECU as a condition where there's variation in detected and predicted timing/speed of the crankshaft and camshaft sensors. So, the engine might feel rough, but it may not be due to a specific cylinder misfire.

Not sure how smart the obd1 ECU is at distinguishing the misfire condition from just rough running.

So, I can imagine conditions where the ECU won't flag a misfire.

cheers,
george.

 

[AaronM33]

Yeah, my 94 had a bad misfire when wet because the wires were shot but no code. I fixed all that and it was great for awhile. Then my intake tube cracked in a new spot and it would misfire when cold on closed loop. Fixed that with more tape and again fine for awhile. Then my HG blew and I have no compression in cyl 6, and it finally threw a code 25.

Moral of the story I think the OBD1 is a little funny when it detects a problem and throws a code. Maybe look at the easy obvious stuff first.

And don't take compression for granted, this is my 2nd head gasket in 2 years (s*** parts). Also FYI, maybe you know already, Code 25 is lean condition, which means there's unburnt oxygen (and fuel, but it's not a fuel sensor, it's an oxygen sensor) in the exhaust stream= misfire.

I would take a look at the ignition stuff, especially the wires. Under load is when they start to short through the insulation.

Fix it and post up a picture of the truck on a beach for us to envy.

 

[AaronM33]

Head gasket is solid. Tune up stuff probably overdue. It just seems strange that it's misfiring but not throwing any codes.

Misfire (at least on obd2 models) is detected by the ECU as a condition where there's variation in detected and predicted timing/speed of the crankshaft and camshaft sensors. So, the engine might feel rough, but it may not be due to a specific cylinder misfire.

Not sure how smart the obd1 ECU is at distinguishing the misfire condition from just rough running.

Looks like OBD 1 uses the same strategy to detect a misfire, bit on OBD 2 trucks they're constantly being monitored for a misfire and this looks like it's 2 trip detection logic, so it would have to detect a problem in 2 drive cycles. I'm asking as much as I'm telling.

Also, not really the OP's question but for general edification there is no "misfire" code on OBD 1 trucks, 25 & 26 "lean condition... " are it. OBD 2 is more sophisticated with constant monitoring of misfire (doesn't take 2 trips to detect and flag, it's nearly instantaneous) and ability to pinpoint which cylinder is misfiring.

So yeah, maybe don't worry about the MIL for now.

 

[AaronM33]

Also look closely at that FSM page and appreciate the Japanese man (or woman) who translated it into English for us.

Normal Made
Teat Mods

 

[hawaiicruz]

Thanks for all the replies I was on to checking the o2 sensors next and I remembered the wiring for the inboard sensor was always a little janky. I had seen these splices before but just assumed an aftermarket sensor had been used. It never gave me any trouble, so I didn't give it a second thought.
Now I realize these splices were made upstream of the connector, and there is no wire loom around the wires where they go above the transmission, so someone has been in there before me. Also, there is a blue wire that is fatter than all the others, and had a strange splice into the white wire leading into the plug. The blue wire looks like some sort of coax with an inner wire and an outer wire, and it has an abrasion that was covered by electrical tape . I am planning on redoing these connections next but that I have never seen anything like that fat blue wire. I traced it back and that's how it emerges from the main harness. So I am wondering how to make that splice???

 

[Rwhat]

That wiring looks a bit suspicious. 02s are voltage based. So a bad crimp, corrosion, aftermarket O2 could be a possible source.

I highly doubt it's because of the missing drive shaft. (In case someone points that out)

Although I would assume if you had a bad sensor it would go into limp mode. Closed loop and run a bit rich and not pull a code 25 lean. Just my guess.

I would look into a vacuum leak.

What's that connector coming down off the frame rail?

 

[team420]

From my knowledge of obd1 toyotas... 25 is always a lean condition... In other words. A vacuum leak... They def dont monitor misfire the same as obd2... My experience is... "Lean condition" is vacuum leak, or exhaust leak (upstrean from the o2), and "rich condition" is ignition issues, efi issues, or ecu issues...

 

[hawaiicruz]

That wiring looks a bit suspicious. 02s are voltage based. So a bad crimp, corrosion, aftermarket O2 could be a possible source.

I highly doubt it's because of the missing drive shaft. (In case someone points that out)

Although I would assume if you had a bad sensor it would go into limp mode. Closed loop and run a bit rich and not pull a code 25 lean. Just my guess.

I would look into a vacuum leak.

What's that connector coming down off the frame rail?

Thanks R
o2s appear to be OEM I will be redoing those splices today (although that fat blue wire "coax"? has me stumped)
I will check vacuum hoses again.
I had pulled the driveshaft and locked the CDL because the shudder actually felt driveline related, but after reading so many threads about this, and performing a stall test. I realized it is definitely in the engine
That connector coming down is from the o2 sensor

 

[hawaiicruz]

From my knowledge of obd1 toyotas... 25 is always a lean condition... In other words. A vacuum leak... They def dont monitor misfire the same as obd2... My experience is... "Lean condition" is vacuum leak, or exhaust leak (upstrean from the o2), and "rich condition" is ignition issues, efi issues, or ecu issues...

Thanks
After I replaced the cracked intake tube the CEL went away so I ruled out further vacuum issues, but will be checking all vacuum lines carefully.
I'm going to do the plug wires next I have been putting it off because I wanted to rule out the cheap stuff first
I practically have a fleet of cars and it seems like they usually all break at once so I'm always looking to save $ when I can...Maybe its time to get rid of a few... But they will have to pry my Land Cruisers out of my cold dead hands

 

[alia176]

Can you do a "power break" test to see if you can create a misfire or make it worse? Left foot on the break, right foot on the gas, tranny in L. This will torque the crap out of the engine and may induce a misfire. If so, maybe it could be the harness by the EGR area, not sure if that's causing a lean condition or not.

 

[hawaiicruz]

Can you do a "power break" test to see if you can create a misfire or make it worse? Left foot on the break, right foot on the gas, tranny in L. This will torque the crap out of the engine and may induce a misfire. If so, maybe it could be the harness by the EGR area, not sure if that's causing a lean condition or not.

Thanks Alia,
I actually found that in one of your old posts, did it and was able to induce the symptoms. The harness looks fine but I know that doesn't mean anything. I ziptied the harness away from the EGR, and still have the problem. If the plug wires don't work I will be digging into the harness next.
I have been trying to avoid all work that is involved with that (Removing throttle body etc.). Working on that area on a lifted 80 is a major PITA. I am actually thinking about building some sort of platform, so I can sit in the engine bay and work right there

 

[alia176]

Thanks Alia,
I actually found that in one of your old posts, did it and was able to induce the symptoms. The harness looks fine but I know that doesn't mean anything. I ziptied the harness away from the EGR, and still have the problem. If the plug wires don't work I will be digging into the harness next.
I have been trying to avoid all work that is involved with that (Removing throttle body etc.). Working on that area on a lifted 80 is a major PITA. I am actually thinking about building some sort of platform, so I can sit in the engine bay and work right there

Or, just remove the two wheels and sink her to the ground!

Are you sure it's electrical and not fuel delivery related? Does the engine rev normally in N or P? I mention this because lean conditions are caused by low fuel delivery issues (pump, filter, injectors) Your problem just started so I'd hesitate to blame the weird O2 sensor wiring issue at this moment. It's been fawked up for a long time, LOL!!

 

[hawaiicruz]

Lowering the truck is a great idea I cant believe I never thought of that.
Yes the truck revs fine in neutral and park, but I'm actually not sure if it's electrical or fuel related, just the sudden way it started made me think ignition

 

[Rwhat]

I did this when I did my HG. Could have gone lower.

 

[alia176]

Lowering the truck is a great idea I cant believe I never thought of that.
Yes the truck revs fine in neutral and park, but I'm actually not sure if it's electrical or fuel related, just the sudden way it started made me think ignition

speaking of lowering, this is what I do: put the floor jack under the front diff, raise, remove both wheels, then lower it until jack bottoms out. This puts the disk rotors about 1/2" off the ground.

regarding the cutting out issue, let's do some more investigating on that front. It'd be very wise to place a fuel pressure gauge somehow in the fuel system I've never done in on this rig but I think people are using the return line somehow to insert a fuel pressure gauge adapter.

On my 4Runner, I was able to place the fuel pressure gauge adapter thing on the banjo bolt on the fuel rail. I think you can do the same thing in our rigs too. I was able to rent a fuel pressure kit from O'Reilly's auto that had this banjo bolt so you maybe able to do the same in your neck of the woods. This will tell you quickly if your fuel system is wonky for some reason.

Come to think of it, if you buy this banjo bolt adapter thing and find a place in your engine bay, just leave it there for future troubleshooting. I installed this permanently on the 4Runner fuel rail: Fuel rated Schrader valve for testing fuel pressure. Rated for Diesel or Gas.