Misfire and Bogging 94 ka24de 'vert

[amencowboy]

Hi guys, first post here.

I searched the forums extensively for a solution to my problem but was unable to find anyone that had the same symptoms as I am experiencing nor anything that would pinpoint what is definitely causing the misfire so I decided to create this post to document what I have done to fix the misfire and maybe gain some insight into what may be causing the misfire.

1994 240sx Convertible KA24DE Manual Transmission 145K miles.

Upon startup the engine shakes pretty bad and holds idle at about 1100 rpm. When driving it bogs at low rpms in 1st, 2nd, and 3rd gear but picks up and pulls strong at high rpms (above 3000ish) in all gears. In neutral, the engine revs up fine and seems to smooth out (the engine will go above 2500 rpm no problem). I'm not sure how the car ran before I owned it, but since it has been in my possession I swapped in a manual transmission (auto was dead when I bought it). I seem to remember it running smooth and not misfiring when I went to go look at it and the few times I started it before the transmission swap. After the swap a few days later is when it started to bog intermittently upon acceleration. Then it started misfiring at idle on cylinder 2. In neutral, the engine pulls strong and does not bog as it does when in gear and moving. At first it would not always misfire. On a cold start it would run fine, but if the engine got warm and then was turned off and started again without much time in-between to cool down, it would misfire. Now it misfires everytime it is started. If you put your hand on the exhaust you can feel the uneven pulsation. Also, you can smell the car is running rich.

No check engine light.

The misfire is coming from cylinder 2. When the spark plug or injector plug is pulled the engine runs the same, any plug on all the other cylinders effect the car and almost make the engine stop running.

Tried all the normal suspects sparks plus, wires, fuel injector etc.....

Hooked up a noid light to the injector harness plug and it flashed showing that the ECU is sending the ground signal to the injector.
Resistance on all injectors reads between 10.5-11.8 ohms.
Changed spark plugs. #2 plug was black and covered in soot (#1 also).
Good spark arcing from wires to engine block.
Rotor and cap are clean and timed properly.
Engine timing is correct.
Cleaned the MAF.
Installed new fuel injector.

Did the blinking led ecu test and got one long and two short blinks (12) which is mass air flow circuit.

When the MAF is unplugged the engine still misfires but the idle drops down to around 600-800 rpm.

I have ordered new plug wires and fuel filter and I will be looking into the MAF code some more. I read a lot about the intake air control valve causing problems so I am in the process of removing it to bench test and clean. Next up is cleaning/testing intake air temperature sensor, coolant temp sensor, throttle position sensor and fuel pump.

Not fully convinced the MAF is the source of all my problems. When the MAF is plugged in it will rev above 2500.

Would having the original automatic trans ecu complicate things? I know there is no rev limiter because of the manual swap.

I will post my progress as I try and diagnose this misfire issue. Please feel free to chime in with any useful info. Thanks for reading.

[PapaSmurf2k3]

Welcome to NICO!
Thanks for the well-written detailed description of the work you've done so far.
By this time, the wiring harnesses on our cars are getting quite old, so I always ask people to shake the main EGI harness (with the car running) around the TPS/coolant temp sensor area. A lot of times that's exactly where they go bad. If the car stumbles or idles up, you know you've got a problem.

You should also check on the coolant temp sensor, like you mentioned. Look at the connector and verify it isn't corroded. Ohm the sensor out to make sure its doing its job. Lastly, inspect your engine grounds to make sure they aren't corroded or broken. If they are gone, all sorts of freaky crap can happen. There should be one on the back of the head, exhaust side (below the valve cover) going to the firewall, and one from the battery going to the intake manifold (near the throttle body kindof).

[amencowboy]

Thanks for the welcome PapaSmurf. I will definitely try wigging the harness around and see if that effects anything. I had a car I was replacing the piston rings on last month and when I got the engine back together and went to start it was missing on one cylinder. After a few tests and a lot of head scratching someone accidentally bumped a connector and the cylinder fired. The cylinder was not firing because a wire going to the injector was loose in the plug. After repining the wire into the plug the engine ran good as new. Just goes to show how one loose wire can make an engine run poorly.

Yesterday I cleaned up the ground on the exhaust side that was connected to the head and another ground on the front of the block. I will have to hunt down the others and clean them up also. This car is not my daily driver, but I plan to go through everything and make it a solid running car again. After I get this misfire figured out the I plan to replace all the rubber suspension bushings with the Energy polyurethane kit that has been sitting around my garage. The car is remarkably clean (and stock), who ever owned the car before me took really good care of it. No rust on the body (pinch welds are fully intact) and the interior is super clean. I know how hard it is to find one of these cars that has not been modified to someone else's tastes so when I saw this one I jumped on it, even though it had an automatic slush box.

[PapaSmurf2k3]

That's awesome. You should post some pictures

[amencowboy]

Been busy but finally got some time to work on my s13.

Changed the front crankshaft seal (front main seal) because it leaked a small amount of oil onto the harmonic balancer which then flung the oil around the bottom half of the engine. This required removing the fan, shroud, belts, radiator, draining coolant, and unbolting harmonic balancer. It is possible to replace the front seal without removing the radiator, but doing so allows enough room to fit an impact gun on the harmonic balancer nut, plus I had no idea when the last time the coolant was changed and as easy as the radiator is to remove, I probably saved time by not having to wrestle with the harmonic balancer nut with a ratchet.

I removed the IACV which had a nice layer of carbon so I cleaned it. I did not expect this to fix my misfire problem but figured after 21 years it deserved a cleaning. Ordered a new gasket and bolted it back on.

To remove the IACV I pulled the top heater core hose off to gain a little more access (coolant was already drained from the system but the hose sits pretty high and you would probably only lose a small amount of coolant by pulling it off), then I unscrewed the long hexagonal solenoid FICD valve and unplugged it (blue connector). Then I unplugged the brown AAC solenoid and unbolted the four 10mm bolts holding the IACV unit to the engine. It is a tight squeeze, but I got a 1/4" ratchet in there on a few of the bolts but had to use a wrench to reach the rest. There is not much room between the engine and firewall and much wiggling and patience was needed to remove the IACV assembly. I used a pry bar to break the seal from the gasket and get it out from between the bracket that holds the metal tube and the intake manifold. Once free I cleaned with some carb cleaner, a small plastic bristle brush, and some cotton swaps.

The IACV gasket would have probably have been damaged if I tried to squeeze the IACV and gasket in at the same time so I made a note of how it would need to be positioned to line up correctly after I got the IACV in place

When replacing the IACV, I had a hard time getting the assembly between the bracket that holds the tube (which shares the bolt going through the top right corner of the IACV) and intake manifold. I had to pry the metal tube bracket away from the engine to make enough room to slide the IACV against the intake manifold, and then pry again to make enough of a gap to slide the new gasket into place. Aligned all the bolt holes and bolted everything back together and connected the heater core hose (this will cause a small air pocket if you did not drain the whole cooling system and will need to be removed by belching the air out somehow).

Installed a new coolant temperature sensor. When I was cleaning the mess that years of a leaky front crankshaft seal made I found the underside of the coolant temperature plug was coated in dirt and oil. Again, I do not think this was the absolute cause of my misfire, and I know I could have tested the old sensor to see if it was still reading the correct numbers at the specified temperatures, but for how cheap the part is and how important it is I decided to install a new one. No teflon tape on new threads, sensor I believe needs the contact with the metal to work correctly? Coolant sensors play a very important role in the PCM's management of an engine because it uses temperature as an input to many of the functions that the PCM handles. Slight defects in coolant temperature sensors can result in inaccurate numbers being sent to the PCM which can cause the PCM to deliver fuel and air in non-ideal quantities. Temperature is a parameters in the PCM's function to compute how much fuel and air to deliver at any given time. If off by a little, you may not even notice it, off by a lot and it throws the whole game off.

When I went to replace the plug wires I found that the wire between the cap and coil was stuck in the cap. It had corroded and was seized in place. I decided to buy a new rotor and cap. It is important to note that there are two distributor manufactures for the s13, Hitachi and Mitsubishi, and the caps and rotors are specific to both (the bolt holes are a few millimeters off between the two and will not fit the other manufacturer).

I found a large black spot on the air filter which puzzled me for while until I learned that the s13 has a small pump that is supposed to pump air into the exhaust manifold in the event it detects negative pressure. Nissan felt that this air needed to be filtered and connected a hose to the main air box which would feed into the exhaust manifold. This system is completely mundane and was taken out of the design of the s14. With the car running I pulled the hose going to the air box and found that instead of supplying clean air to the pump it was venting exhaust gases into the air box. Simple fix for the moment was to remove the black plastic cube and plug the hose going to the pump with a clamp and bolt. I plan to remove it completely in the future.

So after all of this the car runs a little better than before, misfire is not happening as often and goes away when the engine heats up. It bogs on acceleration at low rpms but comes back strong after about 2900 rpm. The car does run better and idles great when it is warmed up. The new cap and rotor helped clear up some firing issues but probably were not the only cause of the engine running rough and misfiring. Next plan is to change the MAF and see if that helps anything. I was getting the MAF code from the PCM but the engine is able to rev over the limit that limp mode restricts it from revving above and when the MAF is unplugged the engine idles lower and the misfire does not disappear.

[PapaSmurf2k3]

Yup, MAF would be next. Either clean it, or swap in a used one (or borrow one) to see if it clears things up. New they are quite pricey.
You also might want to disconnect the battery for a while so the ECM dumps all its memory (from learning how to run with bad sensors) and resets itself.

[amencowboy]

I cleaned it at some point I trying to diagnose this. Unfortunately, I don't know anyone with a 240sx so I am going to have to buy a MAF. Also was thinking I should check the IAT sensor. Fuel Filter was clean as can be.

Does anyone know if completely removing all the components to the pulse air pump that pumps air into the exhaust manifold will trigger and DTCs?

[PapaSmurf2k3]

Mine certainly didn't.

[blkvrtswp]

Before spending money on a MAF, try checking the MAF to ECU wiring for shorts. Really unwrap it and verify nothing wrong. Shorts in the harness are very common and with the ECU throwing a MAF code it is even more likely.

I've found more MAF wiring problems than bad MAFs over the years.

One other thing to check is injector O-rings mis-seated or damaged causing an internal fuel leak - perhaps you are flooding out cyl 2. Pull the fuel rail off with hoses and wiring attached. Turn key to RUN, fuel pump primes and you should see no fuel dripping. Now crank the engine, you should see puffs of fuel from each injector.

(Disclaimer: I know SR's much better than KA's but most of this is the same for both - I think...)

Good luck!

[amencowboy]

Installed a known working MAF and that did not clear up the issue. I also plugged up the pulse air pump.

Been busy at work and had some issues with my daily driver but things have slowed down so I brought my s13 to the shop today.

Pulled the spark plugs and #2 was covered in soot while the other 3 were normal.

Short and long term fuel trims are running a little out of spec. The levels indicate the engine running a little lean so it trying to compensate by adding more fuel. The long term was around 120 and the short term was at 112.

The O2 sensor mV seem odd. It is hovering around 530-640 mV and not jumping up and down as much as it should be (200-800 mV).

The IAC duty cycle percentage is all over the place. It is supposed to be around 15-25% at idle. It was sticking at 50% but then I adjusted the screw and got it down to the 15-25% range but after giving it some gas it would still be high. Going to look into this a little more.

The timing degree BTDC value was at 20. Not sure what it should be but I am about to get the old timing light and make sure the timing is correct.

[amencowboy]

At about 2500 rpm the o2 sensor voltage would start to alternate from about .4-.8 V. At idle it holds steady at around .550 V.

The fuel injector pulse on all 4 injectors seems to be a little long. Back probed each injector and compared to the waveform to what it is supposed to look like and all 4 waves looked a little wide.

Also ran a leak down test on the cylinders, everything was normal.

Hooked up a vacuum pump to the EGR valve at idle and it killed the engine almost instantly when vacuum was applied to open up the EGR valve which leads me to believe that the EGR system is not the problem. I had read about 3 of 4 passages being clogged and the EGR stuck open and it would kill whatever cylinder had the unclogged EGR passage.

The engine is missing but it is not constantly missing. I'm not even sure if it just missing on one cylinder anymore.

Thinking about getting another o2 sensor and trying it out to see if it changes anything. I don't think it will but at this point I am running out of ideas.

[PapaSmurf2k3]

Maybe injector 2 is leaking. You said it was all carboned up right?

[amencowboy]

I popped out the injectors 2 and 3 yesterday and swapped them. Nothing changed.

I'm starting to suspect maybe a faulty PCM, distributor/cam angle sensor, or maybe uneven flow of fuel injectors.

PCM because the pulse width for the injectors seemed to be really long and the engine is running rich.

Distributor/cam angle sensor is more of a hunch and I don't have anything to back it up other than after messing around with everything on the car at one point I had a cam angle sensor code along with a MAF circuit code. Could not get the codes to return.

[amencowboy]

Just checked all the ground pins on the ECU. Everything is good there.

[amencowboy]

Found something odd. I pulled the insulation back on the MAF harness and found the black/white, white, and black wires. Pulling the insulation back even further I found a 4th brown wire soldered onto the black wire. I followed the brown wire and it is grounding onto the engine block. I cut the extra brown wire off and pulled it out. Going to test drive the car and see how she runs without a improperly grounded MAF.

[amencowboy]

SOLVED!!!!

The improper grounding of the MAF is what was causing the engine to run rich and rough. This was a long journey and really pushed my diagnostic wits to their end. Not even sure why i decided to pull the insulation on the MAF wires that far back, but thank god I did or I would have never noticed the extra wire sliced into the wiring.

In the beginning I do believe that I had a bad injector on cylinder #2. Cleaning the IACV also helped the engine run a bit smoother.

Not sure if anyone else is ever going to have this problem but I hope this post can help someone out.

[PapaSmurf2k3]

That's weird. What does the wiring diagram say about the wire that that one was spliced into (and grounded)? Is it supposed to be a ground?

[amencowboy]

Wiring diagram did not show the MAF being directly grounded to the engine block or any splice in the wire. I am pretty sure all 3 wires end up back at the ECU. Plus it was a sloppy solder job and I could see where they jammed the wire into the harness and up a little ways towards the sensor. Definitely not factory.

When the scanner was hooked up, the MAF readings were within spec. I think the wire somehow managed to mess with the signal just enough that it threw the air/fuel ratio of out wack but the ECU did not perceive it as bad enough to throw a DTC. Car runs a lot better now that the extra ground wire is gone. When I got the car it was throwing a code for the MAF circuit so I replaced the MAF. My guess is that whoever had the car before me tried splicing in this ground wire to trick the ECU or somehow thought grounding the wire would fix the issue. This is why you should not mess with wiring unless you know what you are doing.