Hi All, been a while since I've posted, new house purchase has done it's best to keep me away!

 

The car lately has been idling terribly and stuttering badly when at low throttle and driving. At mid to full throttle, the car runs as well as it always has. all 6 cylinders run smooth at higher throttle.

 

I know the engine has poor compression on one cylinder and has for quite some time, but I'm hoping to find another cause for the terrible low speed running

 

I've finally got round to getting data scan connected and i'm just learning how to use it. I've attempted to adjust the AAC base idle but the car will still not idle with the screw all the way out. Also swapped the AAC valve for a spare with no change.

 

Throttle position sensor seems to read good readings. (0.4 at no throttle, 4V at full throttle)

 

MAF reading is not so easy to read, but the reading is approx 1.5V at a very rough idle and rises with flow as expected.

 

My thought is that the fuel pressure regulator is shot and is over fuelling at idle, so I'm on the look out for a FPR to swap out, and I'll be making a fuel pressure gauge to check pressure.

 

If I force the water temperature reading to -7 deg C the idle comes higher as it should do and it just about stays alive, but not tried driving like this yet. What effect would this have? Just a high idle, or does the fuelling get adjusted with temperature?

 

Does anyone have any suggestions on what to check with Data Scan or other tools? Any help appreciated!

I have Conzult on a netbook which would quickly identify individual problem cylinder(s). Vacuum leaks can cause idling problems and so a quick check of all connections for tightness would eliminate this as a contributory factor (affecting all cylinders). Moving up the more involved scale are the under bonnet electrical connections. Large temperature variation tend to promote contact corrosion and there are plenty of candidates on the Zed. If individual 'faulty' cylinders can be identified (e.g. by Conzult) then if gently waggling the connectors on top of the injectors causes a change in idling this could localize the culprit. The AAIV assembly has a couple of connectors which should also be de-mated and checked for oxidation. Try the easy things to start with, but without something (like Conzult) to localize the focus area to fo it could be a lengthy process of trial and error.

You’ve checked for vac leaks I assume? Try disconnecting the MAF and see what happens. When you say low compression, how low?

Thanks for your thoughts so far, with Nissan data scan I can turn off individual cylinders, but I can't even get it to idle at a high enough rpm without altering the water temp to a minus value at the moment.

 

On boost the car often runs as it should, and pulls well.

 

Low compression on cylinder 5 is at best 70 psi, and had been for a couple of years at least. But not been able to check compression more recently.

 

Yesterday I changed the AICV (Aac) and fitted new connectors to rule out bad connections on that. No change.

 

All injector connectors are perfect, and we definitely have all 6 cylinders further up in the rev range. I have a set of coil pack connectors to change, but there is definitely a change in running when each coil pack is disconnected, suggesting connections are all OK (if a little cracked and broken)

Repeating Stephens' question, have you checked the integrity of the vac circuit? If things improve with higher revs, but worsen with lower, this could indicate a leak. If you have not do so already, try disconnecting vac pipes one at time, smallest ones first (at plenum end). A no change result would identify a problem. And don't forget the larger and significant pipe at the back of the balance tube. BTW I use tie-wraps to secure the ends of the small/medium sized vac pipes.

I'll have another thorough check of all vac lines, and will do a leak test as well. I have a new FPR on the way to rule that out.

 

If cylinder 5 has recently dropped compression all together, could the excessive blow-by be pushing gas back in to the plenum though the PCV valves? I'll experiment this evening with that thought... The engine does breathe heavily though the oil filler cap.

 

 

I am in talks with Jimmer about an engine rebuild as we speak, so this is all for piece of mind that I don't have an external factor that could cause issues with a rebuilt engine.

 

Thanks for your help again! I'll report back my findings.

Probably won’t blow past the pcv and it’s designed to be sucking through them at idle. Does it improve when upto temp? Tbh if your having the block rebuilt I’d not worry too much. Is Jimmer pulling and fitting the block or are you doing that to reduce costs?

The plan is to pull the engine and refit myself. It would be nice to determine the cause, if the low compression isn't it, prior to pulling the engine

So this problem is now solved!

I treated myself to a series 2 PTU and a brand new MAF from Z1, and the idle is better than it has been for at least a year. I think the main issue was the MAF, but the PTU is a nice preventative thing.

 

I soldered in a set of new coil pack connectors which has also Solved the wiggly connector intermittent cylinders.

 

Fuel economy is back to what is was too, winning all round!

Still running very low compression on cylinder 5, but we'll soldier that on for a little while and enjoy the ride...

 

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It'll even idle down at 750-800 rpm, which it hasn't done for ever!

Good stuff buddy

Yep seconded good work perseverance paid off and at not a major cost, win win:thumbup::clap: