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Hi Eric and everyone. This is probably tied to my first post about bad gas mileage. I had to do some work on my G35. Changed the water pump and the timing chain and now I have this one final problem that I have been trying to figure out for a month. My car runs lean at idle Nissan uses A/F alpha settings, which are fuel trim I believe. At idle my car is running at 135%!
I replaced my Y-Pipe, mid-pipe, and had that welded to my muffler. I put some SeaFoam in and didn’t really notice any other leaks.
Cleaned MAF sensor and even bought a brand new OEM one. Replaced two upstream A/F sensors (O2) with used ones and no difference either.
I did a power balance test engine reacted to every fuel injector I turned off.
I went around the intake tube and manifold with unlit propane and no change in the idle.
I hooked up my vacuum gauge to my intake manifold it reads ~22hg pretty steadily, drops down to ~19hg, and then raises back to ~22hg. Now, I have noticed that when I monitor the trim levels during this when the vacuum drops the ECU is trying to reset the fuel trim ratios from 135% to 100%.
I’m not really sure where to go from here or what else could be causing this. Outside of the idle problem and bad gas mileage the car works flawlessly. I have no power loss or anything like that.
Any suggestions? Thanks for reading my post!
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Not sure about Nissans, but some engines have problems with the intake manifold gasket… Especially if the intake manifold is plastic… Can be hard to check with propane….
I’m sure you have checked, but ensure there is no unmetered air getting in from between the MAF and the throttle body…
Also, low fuel pressure can cause a P0171… May want to hook up fuel pressure gauge…
-Karl
Thanks for your reply; I’ll check the intake tube again, but I’m pretty sure it’s all snug. The gasket is also thin metal I believe.
Checking the fuel pressure was next on my list, but I just need to research it a little more. I don’t have a schrader valve on my car it requires some kind of special adapter I had to order. Wouldn’t I have a lot more problems with the car other then at idle though if it was fuel pressure related?
When you are getting to the point you are (replaced MAF, replaced O2 sensors, replaced exhaust components) the list starts getting short of what it can be… So many times it is caused by a MAF, but apparently the car isn’t going to give up its secret that easy…
There can be tiny leaks in the vacuum lines as well, truly the best way of testing is with a smoke machine (which most people don’t have access to)….. I’ll be honest, I have had very little luck using propane to find vacuum leaks… I have found that when propane finds it, it is just as easy to hear it….
You may want to try spraying water around the intake and see if you hear it bog down…. Eric has a video on this where he used a spray bottle to find an intake leak on a Blazer….
As for the idle being fine… Well, it may not be fuel pressure related, but fuel pressure is still something that can cause the issue…
-Karl
I had real bad exhaust leaks, which is why I replaced those. I also had some tears in my vacuum hoses, so replaced them with silicone, lol. I’m just perplexed everything is snug and it’s completely frustrating me.
I’ll try the water method and see if anything changes.
I’m actually building a DIY smoke machine it works pretty well; I just need to plug some holes, so the smoke doesn’t leak out.
FWIW I don’t recommend silicon for vacuum lines….The heat of the engine can soften them up and collapse them…. Regular vacuum line is fine…
I used a party smoke machine to find leaks in exhausts before… The thing to remember is the pressure has to be fairly low or it can damage the EVAP system…
All the best…
-Karl
You said you run lean at idle, what do you do at 2500 rpm? If your STFT drops to acceptable levels and your LTFT gradually drops. You have a vacuum leak. How does your cat look? A restricted cat can also play havoc with fuel trims. Try to monitor the trims at idle and 2500rpm first and let us know what that does. Happy hunting!
I’m using Consult 3 to do all my tests and read the data. I have some screenshots from Consult 3 running at idle and after revving to 3K for 5 minutes.
I went into work support mode set the self learning to 100% and can watch it climb up to 110%.
IDLE graph: http://s50.photobucket.com/user/coffeysm/media/IDLE-1.jpg.html
IDLE: http://s50.photobucket.com/user/coffeysm/media/1.jpg.html
When Idle drops: http://s50.photobucket.com/user/coffeysm/media/2.jpg.html
Revving at 3K for 5 mins:
http://s50.photobucket.com/user/coffeysm/media/AFTER2.jpg.html
http://s50.photobucket.com/user/coffeysm/media/AFTER1.jpg.html
FreezeFrame: http://s50.photobucket.com/user/coffeysm/media/P0171.jpg.htmlThe voltage difference between both sensors bothers me. These were taken before I put the user A/F sensors in, but the results are the same regardless.
Thanks, I bought a whole kit for my car and they came with it. I’ve been pretty happy with them so far, but I’ll keep an eye out if anything weird happens.
I didn’t think abut using a fog machine it would probably have been easier, lol. I have a regulator on my compressor to lower it down to a couple PSI.
I second not using RTV to repair vacuum lines. It’s much better to just replace them. They’re cheap enough.
Actually my first thought was to check the timing. Since you just did the chain, perhaps you’re a tooth off somewhere. It’s not hard to do on that engine if memory serves. That could cause an issue with cam timing that could be interpreted as a lean condition by the computer. It wouldn’t be the first time I’ve seen something like that happen.
Keep us posted.
Hi Eric,
I actually used full silicone replacement hoses not RTV to fix my lines.
Well, I quadruple checked everything before I reassembled it. All three marks on the primary chain were aligned with their respective time marking. Cylinder one was also at TDC on compression stroke when I did all of it. I was also told that on this engine if I was a tooth or more off it would be damaged by now, lol.
When I monitored the car using Nissan’s software everything looked according to spec. It’s just that it loses vacuum from 22hg to 19hg when the ECU tries to reset the fuel trim to 100%. That is also when it runs rough and then goes back to 135%. It’s almost like the engine is saying “Hey, I can’t run smooth at 100% I need that extra 35%”. Everything smooths out when it goes back that high.
I also had this lean condition before the chain was done, I don’t know if I put that in my original message. Sorry if I forgot to.
I’m working on building a smoke machine it’s almost done and hopefully it might find a hole I can’t notice.
The only noticeable difference is that the voltages between both pre O2 sensors is extremely off. That didn’t change either when I swapped them with another set.
Can this be caused by bad spark plugs (thought I’d see rich instead), sticky valves or anything like that?
The only thing I haven’t checked is the fuel pump, fuel pressure, and the actual fuel injectors. If I had a fuel problem wouldn’t it be across the whole spectrum though and not just idle?
Could it be the exhaust manifold or flange connecting to the cat is leaking and I just can’t see it?
I’m 99.99% sure it’s the stupid brake booster or the hard line behind the firewall. I was hooking my vacuum gauge (Thanks for the vid Eric) to the test port on the manifold. Getting those weird readings, so I hooked it directly to the booster port on the manifold and it steadied out and the fuel trims went down 10-20% immediately without having to mess with the ECU. I then reversed the booster line with the check valve (didn’t drive it) and got the same results.
I did notice several months ago that it hisses where my booster is and when I asked my mechanic about it; I was told it’s normal???
A vacuum leak at the booster would do it. Thanks for checking that timing BTW, you never know. In my experience Nissans are famous for fuel injector problems. I’ve run into it more than once. It usually causes a misfire in the cylinder the injector fails in. I suppose they could also account for fuel delivery problems. A power balance test would help you find a bad one. I can’t remember if you’ve already performed one of those or not. If you haven’t, it’s worth a try.
I think it’s also worth checking into the brake booster. Easy enough to just block the port at the engine and see if the problem goes away.
Keep us posted.
Hi Eric,
Yes, I did a power balance test and luckily I have Nissan’s diagnostic software, so I was able to do it electronically. The engine responded to every injector that I turned off. I’m pretty sure I narrowed it down to either the booster itself or the seal between the master cylinder and booster. If I leave the booster completely out of the equation and hook my gauge to where the booster connects it idles normally. As soon as the booster is attached my readings go off the charts. I’ll post back when I fix this problem and hopefully it helps with my gas consumption. Again, thanks for all your vids I have literally watched them for hours and helped saved tons of money!
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