rb25 240sx backfire and hesitation

[S13-RB25]

Recently swapped an rb25 in a 92 240 coupe. Everything went well with the swap and the engine ran great. Increased the boost to 13lbs running real good for 3 months. My engine came with autolite plugs for some reason. So about a month ago i pulled them out to put in some iridium plugs. Ngk bkr7eix-11. My problems started. After that i started getting a slight hesitation at high rpms it progressively got worse and became major hesitation. Backfiring bad while accelerating under boost. No matter the boost level. Sound just like a bee r limiter but it keeps revving. Ive tried another set of coils from raw brokerage, another crank angle sensor from them too. Swapped the autolite plugs back in, grounded the coils, changed the fuel pump, fuel filter and fuel strainer. Tried a new mafs, checked for boost leaks, replaced the tps, regrounded the grounds on the pcm. Ran a seperate relay for the fuel pump, checked to make sure the injectors weren't clogged, tightlened all the pins on the ecu, took out the knock sensors and put resistors in their place. I have good power and grounds to both my coils, injectors, and ecu. Still running the stock Turbo, ecu and mafs. Holding 60 psi of fuel pressure under boost and 44 at idle with vacuum hose disconnected. Mafs reads 4.80 volts at 13psi. O2 sensor is .923 mv at wot under boost. Ive done all i know to do. Any suggestions would be very greatly appreciated. Sometimes it clears up and runs good but its seldom.

[supermario680]

I would go back to standard OEM plugs, they seem to work best, is the same way on my twin turbo 300zx, iridiums are fine for non-turbod cars but seem to not run well on turbo'd cars, maybe try a step colder plug from oem if your going to be running 15 psi or more,

Also check the gap, you usually need a smaller gap when boosting at higher levels too to prevent spark blowout.

[supermario680]

Oh yea and check the ignition timing just in case

[RRRRB]

whats the plug gap?

Im running iridiums with zero issues, just throwing that out there

[S13-RB25]

Currently with the autolites they're 1.1 The ngk's i tried at 1.1 and at .8 seemed to be worse with a smaller gap. My boosted civic doesnt like anything but iridium plugs. Breaks up bad past 10psi with copper cores. I have checked the timing. Time and time again. Lol. I have to lock it all the way over to advance just to get it to 15 degrees. When i eliminated the knock sensors i was able to get the cas right in the middle to get it to 15 degrees. I was leaning towards them being bad because of that but it still broke down and back fired with them eliminated.

[RRRRB]

i had my iridiums down to .023 before i got rid of my misfire at 20psi..

Try and swap in another ECU, not much else left to check.......

[supermario680]

Well iridiums have caused issues on several twin turbo 300z's. The stock gap on the Z's is like .044 but it should be around .035 for running above 15psi.

I admit I don't know the details on RB's but I do have extensive experience with twin turbo Z's which are also boosted high performance Nissan engines from the same era with extermely similar components, so I figured some issues might be similar.

[supermario680]

So did you bypass the knock sensor to get it to 15 deg in the middle? Also have you check for ecu codes? Ohm tested all injectors and coil packs? Compression test?

And RRRRR you said you run iridiums without issues, but then say you had to drop the gap all the way down to the 20's to get them run good.... So that's kinda my point...

Anyways...

[S13-RB25]

Wow that's a small gap. Ill gap them down further and try them again. Was thinking of getting another ecu for it. I had a mines tuned ecu in mind but haven't a clue if they're worth the money. Ive stripped the loom off the coil harness and dont see any problems or bad connections with it. I guess it really has to be ignition related. Thing that gets me is it ran fine at 13lbs til i changed the plugs out and put the iridiums in it. May have been a coincidence.

[supermario680]

Do a power balance test, and clean the PTU connections, maybe a loose connection is causing a coil to drop out and dump unburned fuel, and you should always fully test components such as TPS and Maf through the whole range to see if there working or not, sometimes the "new" one you get is bad too...

[S13-RB25]

So did you bypass the knock sensor to get it to 15 deg in the middle? Also have you check for ecu codes? Ohm tested all injectors and coil packs? Compression test?

And RRRRR you said you run iridiums without issues, but then say you had to drop the gap all the way down to the 20's to get them run good.... So that's kinda my point...

Anyways...

I could get it to 15 degrees but I'd have to have the cas all the way over advanced. With the knock sensors jumped out with 1m ohm resistors the cas would be about in the middle at 15 degrees. Ive tried to pull codes but haven't had any luck. I wired up my cel when i did my harness. It comes on under bulb check but doenst light up when I'm driving. The ecu has the knob to turn but no light. Was thinking if the ecu detected a problem it would light up the mil. Maybe I'm wrong. Once it ran fine but would backfire a couple times right before redline around 6500 rpm but other then that it ran good. Its real random. Most of the time its crappy but every now and then... Gets to fooling me thinking i fixed it. Once it ran good after swapping out the coils. Another time after swapping the fuel pump. And another time after swapping the plugs.

[S13-RB25]

PTU? Ive checked the tps through its full range. Smooth with no drop outs or glitches. Same with the mafs but i guess it may be out of range because i dont know its parameters. I tightened the pins in the coils to the best of my ability and believe they are making good contact. Thought about making a new coil harness. Power balance test? You mean checking compression?

[RRRRB]

So did you bypass the knock sensor to get it to 15 deg in the middle? Also have you check for ecu codes? Ohm tested all injectors and coil packs? Compression test?

And RRRRR you said you run iridiums without issues, but then say you had to drop the gap all the way down to the 20's to get them run good.... So that's kinda my point...

Anyways...

i ran them with a stock gap setting with a stock turbo and 12psi, once i installed a bigger turbo and ran upto 26psi the spark would blow out, it did that reguardless of which plug i used,

both my cars (i have a heavily boosted honda as wel) seem to idle better with iridiums and they last twice as long, thats the only reason i use them.. ive had my hondas plug gap down to .019, its currently at .025 but it now has an aftermarket CDI system

Basicly what im getting at is, its common to have a small gap with a moderate amount of boost

[LSDrift]

Maybe the 7 range plugs are a bit too cold for just 13psi? BCPR6ES seems to be one of the most successful plugs around with the RB25 and are pre-gapped at 0.8mm. How did you check your injectors?

[S13-RB25]

Just pulled them out to make sure the screens didnt have trash in them. I'll check them with a mm. Ill try some 6 heat range plugs but that's what the autolites are. I appreciate everyone's input and suggestions. Ive never built a Nissan so I'm not very familiar with their funny issues. If it was a Honda i would have had it figured out by now cause its just what i know.

[LSDrift]

How are the plugs looking since this has been happening? NGK's OEM recommendation for the RB25 is BKR5EIX-11's which is what I started with. I started having bucking and sputtering issues in higher rpm's after about 3 or 4 months of running awesome and even in summer heat. I switched to the BCPR6ES and added a bit of grounding to the coilpacks as well as giving them a single layer of liquid electrical tape. I haven't had the sputtering issue since. I still have a light misfire when warmed up but I've pretty well narrowed it down to either the injectors, TPS, or CTS, or a combination of the three. Another member on here, Darius, was having similar issues as me and his problem was a single injector gone bad.

[S13-RB25]

The plugs looked good but didnt have them in long enough to really read them. So you took out the iridiums and ran copper platinums? How did Darius determine it was an injector? Ive tried coating the coils in liguid tape before i replaced them. Didnt help any.

[Darius]

Ok since my name is getting thrown in here I'll chime in

First, I've never heard of anyone eliminating the knock sensors on the RB motors mostly because they are a good indicator to use for tuning. I am unsure of why it would affect your base timing either. I would start by putting the knock sensor harness back to stock and eliminating the resistors.

Second, having the CAS turned all the way one direction to get the base timing to 15* BTDC is very common. I think it could be due to the PTU (power transistor unit) = ignitor because my base timing changed when I went to LS2 coils and eliminated the ignitor. Now my CAS is set in the middle. Anyway, don't worry about the CAS position as long as the engine is warm when you set the timing and you can get it to 15*.

Third, the plugs should not have much of an effect on the engine breaking up especially at the boost the car is at. I would suspect the coils before the plugs.

How do you know the MAF is at 4.8V at WOT? Do you have a wideband O2?

For the record, I knew I had a bad fuel injector because my wideband started showing lean (16+ at idle). I don't know if it was breaking up up top because I did not push it when I recognized there was something awry. I tested spark and it was fine so it had to be fuel related. The injector didn't have continuity between the two pins which meant it was burnt up.

[LSDrift]

Haha I figured if I mentioned your thread Darius the OP would search for it...or you would chime in haha. But OP, BCP6ES are just coppers, no such thing as copper platinums, but they're like 3 bucks a piece.

But the most important factor I'd mention here in terms of the plugs is the heat range at your boost/power level. You could be just a hair too cold...you say you were running decent for 3-4 months ago, in what was likely warmer ambient temperatures than now (depending on where you live), so regardless if that's your problem or not having the proper heat range of spark plug is a must. Just my 2 cents...

[S13-RB25]

I hooked the knock sensors back up already. I was using the resistors to eliminate the k.s. as being the problem. Ive replaced the coils already. Yes it was a used set but i feel sure it would have changed it. I have a series 2 engine with built in ignitors. All the injectors have the same impedance. Currently dont have a wideband but i monitor the o2 sensor Mvs which i know is not the best way. I get approx .92 mv at wot when it breaks up. I know the mafs voltage is 4.8 at 13 lbs/wot because i monitor it at the ecu with a multimeter while driving. I dont have a consult and didnt wire in a diagnostic connector... Could have sworn ngk made a copper core plug with a platinum tipped electrode. My mistake.

[S13-RB25]

I pulled the plugs to try some bcpr6kes. Noticed every one of the old plugs were wet with gas when i pulled em. Some worse then others. It was sitting for at least 4 hours. Perhaps i should try to replace the o rings on all the injectors. Or just replace the injectors. Also noticed the porcelain part of the plug was taller these then the autolites and ngk iridiums. I'd think it would make better contact with the coil. I dont think the plugs are going to fix it now that see fuels entering the combustion chamber without the car running.

[supermario680]

#1 - the knock sensor if NOT working correctly OR sensing knock WILL retard the timing to protect the engine. Even at idle, it makes perfect sense that when bypassed you can easily get timing to 15, but when disconnected you need to fully advance it to get it to 15. If you STILL need to fully advance it when its plugged in then it is faulty OR detecting knock.

#2 - the only disadvantage to running colder plugs is that they can foul easier for daily driving aka not high boost/heat conditions

#3 - by power balance I mean start the engine and pull either a coil pack or injector connector one at a time and listen for the rpms to drop engine shake etc.. if you pull one and DO NOT HEAR ANY CHANGE, then that cylinder was never firing in the first place.

#4 OHM test your injectors to make sure they are all in spec and open or short circuted.

#5 - PTU = power transistor unit = sends the signals to the coil packs to tell them to fire. they are notorious for causing issues on 300z's. It is the black square mounted on the top rear of the valve cover on my series 1 RB25.

#6 - don't bother making a new coil harness UNLESS YOU DO NOT HAVE SPARK, and even then, 99% of the time its a PTU issue (on Z's anyways, very similar)

#7 - ECU codes do not always trigger a CEL. I've had numerous fault codes on my Z and not one of them has ever triggered a CEL/MIL etc...

[supermario680]

EDIT: skip 4 and 5, i just saw that you already checked your injectors and IDK if the series 2 has a PTU...

anyways, visually check for spark on all 6 before custom wiring anything, (and do all the other steps...)

[S13-RB25]

I appreciate everyone's help on here. Ive determined it to be the injectors. They all test the same impedance but they're not flowing like they should. Well, not all of them anyway. Just got to find a set now. Think I'm going to spend the extra money and get a set that's been flow tested and matched. Hope i didnt cause any damage to the ring lands which knowing my luck I did.