Misfire, suggestions? going to check it out tomorrow.

[240z4u]

I put a new set of 550cc venom injectors into my rb25det S2 a couple weeks ago. I compensated for them with my power FC, so no problem there. Ever since the installation I have had a rough idle (with a misfire feel to it).

I lightly oiled the orings before installation, and am pretty sure none got into the injector because I was careful with them.

I know some do not like venom injectors too much, but I am not sure its the injectors fault (at least I am giving benefit of the doubt).

When the motor misfires, and this can literally be on every stroke at some points (much worse when the engine is warm). The A/F gauge jumps around a bit, and sometimes sweeps very smoothly back and forth in an 1.5 afr arc (it sways to the rich side). It rarely stays at closed loop. On occasion my egt probe will bounce up a hundered degrees or so too. It is located right between cyl 5/6.

This is all at idle btw. I tried dropping out injectors one at a time to see if the stumble became more consistent, and they all seem to react pretty much the same. Sticking a finger on each one, the solenoids feel to be moving freely.

Just to preface this issue, one time a couple weeks ago my engine felt like it dropped a couple cylinders for a minute or so under boost. I came to a stop, and it was idling super rough (dead cyls it felt like) but then all the sudden everything smoothed back out. This was before the injector swap.

Here is what i am going to check;

Pull plugs to look for an oddball looking one (and replace them)

Check for vac leaks between the plenum halves

Check to make sure I was 100% consistent wiring up the injector harness with the new plugs, I may have flipped the wires on an injector (I dont know if this would make a diff but Ill check).

Do you guys have any other suggestions? Thoughts?

Thanks - Evan
Modified by 240z4u at 6:38 PM 5/24/2006

[Carl H]

is this a rb25 s1 or rb26?the ignitor might be taking a ****, erratic idling and dropping cylinders while driving is a symptom of a dying ignitior.

[240z4u]

Duh, leave it to me to leave out the important stuff. Its a series 2 RB25det.

Ill edit the post above.

Thanks Carl - Evan

[240z4u]

Well, still having the same issues.

Double checked injector harness wiring, its fine.

Replaced gasket between plenum halves as well.

The misfire is at all RPMs and still at idle.

I also checked plugs, and saw no obvious reason there either.

Has anyone had this type of issue with the rb coils? Its almost a studder, not a misfire. Seems to be popping out the exhaust almost (like a bit of fuel lights off in the cat). I can take a sound clip if that would help.

Can anyone offer anything? I Have it narrowed to coils but now I need to make sure its the problem because I do not have a ton of extra $$

Thanks - Evan

[Bluefire]

Ok you need to explain exactly what you modified on the motor prior to the misfire problem. As far as I can see all you did was change the injectors and then had a misfire problem. Now was the car running good for awhile after the injector change or did the misfire occur immediately after the injector install.

Also you can isolate the misfire by pulling the plugs on the coil packs one by one or injector plugs one by one. If you don't notice a change in the motor when you pull a plug that cylinder is the one that is not firing. Now swap that coilpack with another one and see if that cylinder is now misfiring. You can do the same with injectors.

IMO if this happened after you installed the injectors, there is 99% chance it is the injectors. And depending on the quality of the injectors, there might be a big difference in flow. One might be flooding a cylinder or just not flowing enough.

How did the spark plugs look, be specific. What percent are the injectors set to on the power fc... What else did you remove while changing the injectors...

-Bluefire

[240z4u]

Thanks for the reply! It makes life easier when you have people to bounce scenarios off of. Here are your answers, and the picture attached is of my plugs that I pulled out, number one is on the far left.

[QUOTE=Bluefire]Ok you need to explain exactly what you modified on the motor prior to the misfire problem. As far as I can see all you did was change the injectors and then had a misfire problem. Now was the car running good for awhile after the injector change or did the misfire occur immediately after the injector install.

Modification.. changed out fuel rail to a top feed unit and installed 550cc injectors. I also had to change a few fittings on my FPR etc. so everything would work correctly. Nothing out of the ordinary there. My fuel pressure is rock solid at 42 base with line off, and the aeromotive FPR seems to be reacting fine to pressure/vac (this unit was on with the stockers too).

Also you can isolate the misfire by pulling the plugs on the coil packs one by one or injector plugs one by one. If you don't notice a change in the motor when you pull a plug that cylinder is the one that is not firing. Now swap that coilpack with another one and see if that cylinder is now misfiring. You can do the same with injectors.

I did pull plugs on the coils, and could not find a definitive problem cyl. My best job of narrowing it down put me at cyl number 3. The porcalin (sorry for the crap spelling) is more grey than the rest. I also dropped cylinders with the power FC injector map to try to isolate it.

IMO if this happened after you installed the injectors, there is 99% chance it is the injectors. And depending on the quality of the injectors, there might be a big difference in flow. One might be flooding a cylinder or just not flowing enough.

Yes this happened directly after the fuel injector installation, however the day before or something like that my car dropped a cylinder or two for fun after getting on boost. I am still skeptical that it may be coincedental. The guy who I got the fuel injectors from claimed that they were flow tested and matched. I am going to email him, he may be full of crap.

How did the spark plugs look, be specific. What percent are the injectors set to on the power fc... What else did you remove while changing the injectors...

I want to say the percentage on the power FC is like 72 percent, when I did the math it came out to 68 or something like that but my fuel maps were SOO far off I changed the correction percentage to get me in the range. I have no idea what the injector lag time is, but the car runs best with +.08 lag time dialed in.

I removed intake pipe from TB, coolant lines to TB, throttle cable, upper plenum, a couple emissions or whatever lines from the bottom of the plenum, and all the vac lines to the charcoal canister etc... I also unplugged the IAC solenoid connections, and the TPS connectors. Everything got hooked back up correctly, I triple checked, and also triple checked again today when I replaced the gasket between the halves.

Also the injectors all metered out at the same static resistance, and I had put different injector plugs on to accomidate the new injectors.

This is about all I can think of right now though! Ill give it some more thought and post again if I think of any other oddities.

Car idles and revs perfectly until underhood temps start to rise.

Evan

[Bluefire]

Ok you said it dropped a cylinder or two the day before can you elaborate on that situation? Was it a momentary thing and seem like it fixed itself. Have you done a leak down test? Visually check the coilpacks and resistances? ...

[240z4u]

The dropped cylinder was very wierd. I rolled up on boost 11psi, and definitely way rich afr, I was watching my gauges. Low EGT as well.

Basically it started to misfire, really chugging (realtively low RPM 5k). So I got out of it and shifted into 4th. It was misfiring really bad. It would not idle unless I kept the throttle open a little bit with the gas pedal. I sat at a light for about a minute, and then pulled across the street. By the time I got across the intersection + about 1 block the idle went back to normal like nothing happened. I took off and no problems.

I have not done a compression/leak down. I have the tools to do so, so i will do that just to eliminate one more thing.

I took my coils off and examined them closely. There is evidence that a few coils have been leaking at one point or another (black brackets with carbon deposits/and the head had the same right across from the brackets on the head).

I did some reading on skylinesdownunder.com and found that the most common cause of this situation is coils/ignitor. They described the situation well, basically motor is cool and it runs like a kitty, and as it warms up it starts to miss worse and worse.

I also read on skylines downunder that measuring these coils rarely gives any sort of definitive answer. I was also unable to find by searching exactly which terminals to meter for resistance. If you know Ill definitely try it again.

The misfire I am experiencing now is nothing like when it dropped the cylinder the first time. It is almost like a weak spark or something because the motor does not have a hard miss, its more of a stumble almost. I can take a sound clip if that would help. The stumble is so light that its totally masked when you pull an injector/coil wire off due to the extreme misfire created.

Evan

[Bioxide02]

When my car was missing like yours I just went ahead and order'd up some splitfire coilpacks.. The car runs so much better now

[Bluefire]

For the coil packs, try wrapping each one up in high quality electrical tape. There might be some small cracks that might open up more once warmed up. This could also explain why your not getting a full cylinder misfire and more of a weak spark...

-Bluefire

[240z4u]

I wrapped all the coils in 3m super 33+ tape.

Misfire is all but totally gone, I snagged more coils off ebay tonight for 75 shipped with harness.

Thanks alot for your help/suggestions!!!!

Evan
Modified by 240z4u at 8:02 PM 6/6/2006