Help, Motor is Misfiring

[wrcbonk]

Guys,

 

My r33 rb25det has been running (on jack stands with no exhaust) for some time now as you may know. Well the other day I drove it over to the exhaust shop to get the exhaust done. It was a loud and exciting trip in morning rush hour traffic. Luckily the cops were huddled somewhere else. I asked for simple and quiet and thats what I got, nothing fancy, 2.5-inch from the j-pipe back to the muffler (magnaflow), traditional path of the exhaust.

 

The one issue I had with the car before taking in was that it was a missing. I was hoping the exhaust would take car of that, but it didn't.

 

OK before anyone says, "DUDE where's the UTUBE, this thread is worthless without a video!" I know I know. This first post is just a primer. Tonight I will go home and post a vid of the engine at idle and maybe a road test.

 

So, what have a I done to try to fix the issue:

 

1) Replaced the Spark Plugs. I read on a 240sx thread that said worn iridium plugs can cause missing (I left the iridium plugs that came with the motor in, thinking "no need to change plugs until I have a problem"). I repalced with the factory recommended NGK PRF5G-11s. Still a miss. I DID NOT gap the plugs yet. I don't want to be a kill joy and stop this from turning into another "plug thread", but try to control yourselves:)

 

2) Set and checked base timing to 15 BTDC twice. I read the FSM and did the following, warmed the car up to get revs as close to 650 +/- 50 as possible, unplugged TPS and used timing light to check timing. I did remove the CAS during the install to repalce the timing goodies so not too surprised that the base timing was way off the first time I checked. But now I have checked it twice and it is 15 BTDC. And the engine still misses.

 

3) Ignition malfunction/spark in wrong plug coil - We pulled the ornament turned off the lights and missed water over the top of the plugs hoping to see a spark/arch from one coil to another. It was a long shot, gained us nothing, but worth a try. And the engine still misses.

 

What I am planning to do next:

 

1) Run some kind a high test fuel additive/cleaner threw it to make sure the injectors are clean. Rejracer has a gallon of Xylene or some other dangerous chemical he has offered to let me use.

 

2) One night soon, pull the plugs and run a compression check.

 

Like I say I will be back later with a video/audio of the miss.

 

Thanks

 

Bill

[rejracer]

On first read of the title I wanted to respond "well where did it go, it was there last night?"

 

The engine is backfiring through the exhaust. This to me indicates unburnt mixture is igniting in the exhaust.

 

Toluene is what I was going to use, it's an excellent fuel system cleaner. As of now I suspect it's either an ignition issue or an injector spray pattern problem. The plan is to do the following in an attempt to collect info so we can identify the problem.

 

1. check coils for signs of carbon scoring indicative of a failing coil. (free)

2. compression check (Free)

3. "read" the plugs, check gap. (Free)

4. check for exhaust manifold leak (free)

5. check exhaust manifold temps with pyrometer, looking for temp variation. (free)

6. fuel injector cleaner (12.00 for 1 gal of toluene )

7. check injector spray pattern

8. New 02 sensor ?

 

I'm open to suggestions on how to narrow it down.

 

-Robert

Edited March 1, 2012 by rejracer

[RS Speed]

have you pulled coilpacks one by one to pinpoint the problem to a specific cylinder?

 

At least if you can find the culprit you can swap coils/injectors from other cylinder to rule out a bad coil or injector.

[wrcbonk]

RS,

 

Excellent suggestion, we will try that out.

 

Here is the video I promised and two pics.

 

 

 

 

I checked the timing again. Its set to 15 BTDC when in open loop, but I don't like the fact that the CAS has to be turned to near its limits to get the setting. When I bought the motor the CAS was set in the middle of the gaps. Not sure this means anything, hoping the timing belt is not a tooth off. Just had to throw a pic in of my new timing light. Things have changed in the last few decades, looks like something from the set of Battle Star Galactica.

[dexter72]

If the mis is constant then as was suggested above, You have to narrow it down to which cylinder is causing the mis. If you can get to the injector connectors easily. Remove one injector connector at a time and then reconnect it, to see which cylinder with the F.I. connector removed, does not change engine rpm at idle.

 

The cylinder that doesn't change is the problem, Then try swapping coils, plugs, injectors and so on.

[ZT-R]

Yup what those two just said. On a side note, you are checking your timing with a light and not with just what a engine management is saying what it is, correct? Not that that is the issue just saying.

[wrcbonk]

ZT-R,

 

Yes, I used a timing light only to set the base timing, the video was confusing.

 

Bill

[rayaapp2]

I was going to mention to check the timing with the inductive light you have there as well, but its looks like you got that covered.

 

Try pulling injector connectors not ignition coil connectors. It almost sounds in the video as if your lighting unburnt fuel off in the exhaust system. That may just be the audio quality, but its probably a better place to start than the coils to determine the cylinder.

 

My personal advice is to ditch all iridium or platinum plugs. Dont waste your time with them. If they are not causing an issue now they WILL be an issue under boost. The same reason why you would not use bosch quad tips or E3 plugs. Some people swear by these. 'MY PERSONAL OPINION' is that no forced induction vehicle should use any of these plugs. You either do not have enough surface area to work with or far to much. Neither in again my opinion is good. This is my opinion because I have seen the results on a dyno. I have not tested every forced induction motor ever made so there may be some out there to prove me wrong, but I can tell you I have seen this on several RB power plants.

Go buy the plain nothing special copper core spark plugs. It doesnt matter if its the bottom shelf brand. For the RB I special order(because no one carries them in stock) NGK copper cores. Not V power or what ever, just plain copper cores. For stock boost gap those plain copper cores around .035-.036". I went a step colder than stock, but Im sure the stock heat range will be fine for a stock engine like yours.

 

If your not lighting fuel off in the exhaust Id think you have a clogged injector thats way to lean on one cylinder. Once youve sorted everything out as far as the timing and spark plugs do a 'power balance test" which is basically pulling injector clips one at a time and noting the drop in rpm for each cylinder. Once you find the cylinder with the least drop you have your culprit cylinder. Then insure you have power and ground at the injector. Consult a wiring diagram here and see what gets switched power or ground and test them. I believe you should have solid power and switched ground, but check to be sure as I work on a lot of cars and who knows what Im remembering. With the car running you can use a multimeter to check for switch ground(if the case may be as I suspect) and solid power. Or if you have switched power all you need to do is verify ground and use a test light and watch for it to blink at you. Once you have eliminated that part you should be able to move onto the injector itself. Swap that injector with another one. See if the miss follows that injector. If it does you can try and back flush it, but Id just replace it. That doesnt sound like the typical coil pack missfire, but I guess as a last ditch effort you can move onto the coils and try swapping that cylinder coil with another one and again using the injectors cut fuel and see if the miss follows the coil. IF you get your spark plugs changed and youve eliminated the fuel and spark then its mechanical...

 

The only thing Ive missed here is random misfire. If its a random misfire then your gonna have get a bit more involved with your diagnosis.

 

Hopefully that helps Bonk

 

Good luck!

 

Ray

[wrcbonk]

Ray,

 

Thanks for the write up. Robert and I are going to start in on checking the fuel injectors tomorrow. Today we did a compression check, did a visual on the new plugs and the coil packs.

 

Here are the compression numbers, they look good.

 

Cylinder 1st compression 2nd compression final compresion

1 90 120 160

2 85 115 150

3 85 115 150

4 90 120 150

5 85 115 150

6 85 115 150

 

Here are pics of the plugs (1 through 6 left to right), 1 and 2 were discolored, Robert thinks cylinders 1 and 2 may not be firing right. We agree the miss sounds like unburnt fuel ingniting in the exhaust.

 

 

 

 

 

 

We started the engine again after gapping the plugs and the miss is still there, but less frequent. That could simply be that the car was warm. I will start tomorrow cold and hear then.

 

Thanks all

 

Bonk

[ZT-R]

I run bkr7e in my 26. Dont run plats as mentioned.

[rayaapp2]

Dont gap platinum tips either. Gapping them will loosen their platinum tip from their seats and they will FALL out and float around in the chamber until they are spit out or embed their hot bodies into something unpleasant like a valve seat or piston(and they can burn their way all the way through a piston). Even if it spits out the tip... its headed to the turbo turbine. Either way chances are your bound to have a really bad day. Just in case you were not privy to that info.

 

Something fowl is going on with #1. Here is where I would start to suspect something mechanical. See how its fowled up on one side... Thats usually indicative of intake charge blasting the plug with oil. Its possible something else is occurring, but in most cases I would suspect valve stem seals. The discolor of #2 looks slightly wet like fuel fowl. That may be your mis-fire. Let us know how your diagnosis comes along.

 

Ray A.

[rejracer]

If you check out the pics, the 1&2 spark plugs are showing these anomalies.

1. The flat surface ring of the chamber end is showing no signs of combustion. We did not clean the plugs. Notice 1&2 are much cleaner.

2. The ceramic insulators are darker in color.

3. #1 insulator had black smudge on it.

 

Other notes:

A) #1&2 had no fuel smell to it when we pulled them. Car had only been off for a few minutes before we started pulling the plugs.

The piston tops were darker and looked wetter than the other 4 cylinders. Cylinders 4-6 had a brownish tint to them, and 1&2 were much darker and a bit oily.

C) Bill's cold start test will give more insight. If the miss is just as bad, then I'll lean towards fuel, if it's slightly improved we can further inspect ignition.

D) The vehicle does not smoke. Exhaust smells rich, not oily.

 

I gapped the plugs, did not know about the platinum tips! Sorry Bonk.

 

When Bill replaced the plugs the miss was there immediately on start up. Knowing we have good compression that says there is a fuel or ignition issue. At this point I don't believe the engine mechanical is causing the misfire, although there may be an issue that needs correction. Without proper combustion a cylinder will get oily. So my advice it to continue to work the miss issue and then after X miles, recheck for possible excess oil in the chamber.

 

My revised theory:

1&2 are dead, and as such are pumping oxygen into the exhaust. The 02 sensor is picking up this 02 and enriching the mixture. 3-6 are running rich, very rich. when the 02 from 1-2 mixes with the unburnt fuel from 3-6 an exhaust backfire happens. The only thing that is throwing me off is how smooth the engine runs. If 1&2 were dead, I don't think it would not be so smooth.

 

-REJ

[wrcbonk]

The plot thickens. I started the car last night to see if the missing had changed. I started the car and it ran for 15 to 20 seconds and slowly died. I tried starting it again, it ran for 1 to 2 seconds and died. It will not start at all now. The battery is strong, providing more than adequate juice to the starter. The fuel pressure gage is reading 40 to 50 psi, it runs at 40, so I don't think fuel delivery is the issue. I tested the loom side of the CAS and it has power and ground ok. I am sure this is an ignition problem. I got two codes from Consult. Forgot to write them down, but this is what I remember. I will try to get them to come up again tonight.

 

20 - ignition signal failed

 

19 - timing signal failed

[wrcbonk]

Here are the right ecu codes.

 

CODE 11: CRANKSHAFT POSITION SENSOR CIRCUIT (8 STARTS AGO)

 

CODE 21: IGNITION SIGNAL CIRCUIT (27 STARTS AGO)

[wrcbonk]

And the coil plug wires for cylinders 1 and 2 were backwards. THe car now runs and there is no miss. I am going to put a few miles to burn off any left over oil and gas and change the plugs to copper.

 

THanks for the help guys.

 

Bill

[rejracer]

I am surprised how smooth it ran with 1&2 not firing.

 

Exhaust smelled like it should without a cat under it. Exhaust pipe was clean, no heavy soot on it.

Boosted 1 lb higher than last test.

Power is up

 

We will learn more from future plug readings.

 

Other notes:

I did not expect the stock RB25 turbo to spool up so fast. Surprised how little it had up high, but this may change once the car gets more dialed in. I had a T3 in my ZXT when it was a car and not a engine sitting in my garage, and I don't recall it spooling up that fast.

 

I sense a drag race between 2 Z's in the near future.

[ZT-R]

Glad ya got her sorted out.

[wrcbonk]

I am thread jacking my own thread, sure thats nothing new.

 

The missing is generally gone. There is a slight studder/oscillation in the exhaust note when the car is first getting started at low revs. Then when I ring the engine at high revs there is a missfire right when I let off to shift. A friend says it sounds like the engine is running out of air at the highest rpms.

 

My current air intake setup is less then desireable, the only thing missing from the pic below is a cheapo green mushroom shaped filter. The filter sits down near the bottom of the radiator and the front body pan. I think improving the intake may need to be my next project.

 

 

Bonk

[rayaapp2]

That looks okay to me.

Unless you have a kink in the inlet hose or your BOV is not recirculated correctly I dont see an issue.

Is your PCV system also hooked up in there?

 

What do you have to watch lambda? No point in guessing around unless you know you have an air/fuel issue if you cannot sample the actual lambda. Otherwise you will be chasing your tail here.

Id suspect a coil issue still with a miss like that. The exhaust will load up with partially/unburned fuel and light it off when you let off for the shift. Or if you are excessively rich/lean the same thing will happen. The ONLY upside to that is that its kinda cool to see the tail pipe light up your rear view mirrors at night. But the downside is the miss and possible engine damage. The fuel delivery system in the Z even modified isnt really adequate for EFI. Some will contest that statement, but what I found is that the tank needs a sump, the lines are to small, and the stock FI for the RB has a modulator unit for the fuel pump that almost no one integrates into their swap. So no matter how you cut it you will end up with tune issues of some sort. An adjustable FPR helps, larger lines of at least 8mm are needed(I went with 3/8"), a proper tank or surge tank with lift pump, and a tunable ECU or piggy back will all help. You can get away without some of that though and its been done, but that is my recommendation even for a stock RB swap. The volume delivered through 8mm lines with a proper sump pickup and an adjustable FPR were just barely adequate for my stock RB on a dyno. My stock setup went lean on the dyno after 4000 rpms. I used a SAFC II to correct and then found issues with my coils and plugs. It all boils down to our Zs having a completely different fuel system than the donor cars for which the engine was designed to work with. Even a 280z fuel system would have issues as I found out. Its a matter of fuel volume at pressure that you are trying to match.

That should give you some to think on.

 

If you do not have a current way to monitor your lambda that should be your very next step. Either a calibrated wide band or dyno run. OR if you know a local smog tech that will let you sample your car in a manual testing mode you could do a five gas calculation to find lambda and get a general idea.

 

I would say get your engine heat soaked after that. A hard run down the freeway aught to do it. Do this at night. Pull over in a dark place and have a spray bottle handy. Lightly mist your coils and see if any of them arc to ground during your idle miss. I know you mentioned you tried this once. The arc is hard to see, but you should at least able hear it arc. Its important to try and simulate a loaded situation with the heat soak and mist though. If its not hot it may not try and arc and you will still have an issue that you cant find. There shouldnt be any arcing anywhere near the coil. If there is any its done and time to replace.

 

 

Ray

[wrcbonk]

More stupid fixes or fixing stupid. As I reported the harness ends of the coils were switched on 1 and 2 causing major problems. We "fixed" that by switching the two around. But it still was missing as Ray heard last weekend. I was taking the ornament off to start pulling plugs and discovered the number two end of the harness was over but not snapped into the coil. We got so excited last time when we found the problem we didn't take the time to make sure of the connection. We pulled the plug and it looked brand new, because it was, it had only been exposed to fuel no spark. After plugging everything back in we started it. WOW as you might imagine its a new car, no missing. It will still backfire after shifting at high rpms. Robert doesn't think this is much to worry about.

 

Learning

 

Bonk