Adventures with rough idle and misfire

[DatsunBanana]

Had an interesting couple of days. Have had a high revs misfire and rough idle but without any error codes on my R34 (RB25DE+t). Had some used Benchmark coil packs already so fitted those plus brand new plugs which fixed the high revs issue but still had rough idle and now it started throwing code 21. It felt like failing (but not completely failed) coil pack misfire so started swapping one at a time for one of the original coil packs (hoping I was using a good one) but still getting rough idle and code 21. Revs above idle were smooth. I tried Datascan to switch off cylinders but couldn't narrow it down. Decided at that point I may as well try changing the coil pack loom as I've had a spare engine sitting in the corner of the garage for a couple of years. I whipped off the cover and to my delight found a set of Splitfires. Fitted those and the loom but it was still idling rough (but at least no code 21 anymore). Took it out on the road and it drove lovely. Stopped at the side of the road and the idle was smooth as anything. Got home onto the driveway and it was rough again. Started wondering if my steep upwards sloping driveway might have been causing the rough idle all along. Although it seemed like a crazy idea I drove back onto the road ... smooth idle. Back up the driveway ... rough idle. Repeated a half dozen times and it consistently idles rough with the nose pointed up the drive and idles smooth on the flat.

Anyone ever heard of anything like that before? All I can think of is fuel pressure. I've got a Walbro but I got that second hand so could be tired. I've been doing these tests with 3/4 to 1/2 tank of petrol but could that moving to the back of the tank affect pressure? And why would it be fine at anything above idle? Any other components or internals that could be affected by an upwards incline?

Oh and from a cold start, for the first few seconds it's quite smooth till the revs settle a bit then the rough idle becomes noticeable. It's roughest while cold but less so when it's warmed up.

Edited June 2, 2020 by DatsunBanana
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[GTSBoy]

I would suggest a few things;

Do not make any presumptions about fuel pressure without a fuel pressure gauge fitted. It is so easy to do, and answers so many questions or at least stops you worrying about things that are not real.

A vehicle posture cause for rough idle is far more likely to be some air leak somewhere, opening up at that angle, closing up when flat. Or maybe a PCV valve or something flopping into a stuck open/closed position.

[tridentt150v]

Mine is like that when the front wheels are on the ramps.  Soon as I take it off the ramps its fine.  Been like it for 15-20 years.  Was absolutely f'ing angry when it first happened.  But as soon as I got it off the ramps...problem solved.  Never really cared, never looked for a reason, no issues driving or idling.  I probably should......but my roof only leaks when it rains, so stay outa the rain and its no probs.

[DatsunBanana]

Thanks for the replies. I'll have a look at the PCV valve and hoses etc. but yeah, it's not a major worry ... I'm still smiling at finding a 'free' set of splitfires ?

[Stepho1337]

Been through about the same with my Skyline. Try hardwiring your fuelpump for starters, and fuelpumps do go bad, so might also be that. You running stock injectors?

[DatsunBanana]

Yep stock GTT injectors and ECU on a DE+t conversion. Would the two stage fuel pump relay even exist on what was originally an NA car? 

By the way, the PCV hose was split at the inlet end. Swapped that and the grommet in the cam cover but it's made no difference. At least I know that's done now though. 

What about the AAC valve ... anything in there that could flop about on a slope and cause rough idling?

Any other ideas are welcome ? 

[Stepho1337]

I guess if you upgraded the pump, but I’d do the hardwiring anyways. Probably stupid question, but the fuelpump sits in its bracket mounted correctly inside the tank? Ive seen pumps not being mounted properly and just lying free around inside the tank since it can be hard to put it back correctly. All of the cylinders are running?

[DatsunBanana]

Interesting point about the pump. Here's a pic from another thread showing what I had to do to mount it in the cradle. I think a Walbro 342 would have fitted better than a 341 but I tried to make sure it was secure. But in theory the hose clip could have slipped and who knows! Maybe I'll check that. Ordered a fuel filter too as it's about time I changed that.

 

 

[DatsunBanana]

Fuel filter made no difference. I might just let it go now and not worry about it as the issue is negligible. Or if I can be bothered I might check the pump is still mounted in the tank ok and might swap the AAC for my spare cos years ago I lost the spring while cleaning it and replaced it with one from a ball point pen ?

Anyway re. the direct voltage conversion, I checked the wiring diagram and thought I'd post this in case anyone else is interested. I believe that stock GT wiring will run the fuel pump at 13.5V as standard. In the attached wiring diagram, red lines I've marked are all battery voltage from the ignition switch and I've taken a confirmed reading of 13.5V at the solenoid plug I've marked. The fuel pump dropping resistor (and control module) don't exist on the non-turbo so the other side of the pump is direct to ground (green line on diagram). So although I haven't measured it directly at the pump, I reckon that means a non-turbo car will already have full battery voltage at the pump. Happy to be corrected if I'm wrong though.