Guest 2slo4u Posted October 4, 2004 Share Posted October 4, 2004 Hey guys and gals. Anyone know how small an exhaust leak can be and cause erratic idle? I recently (a few months ago) made the mistake of letting a local machine shop (rhymes with larry's) surface my exhaust manifold before I installed my head. I was kind of unsure about doing this because the intake and exhaust manifold share the center row of bolts, but the guys at (the place that rhymes with larry's) said it would cause no problems and that they have surfaced these maifolds before. So I said ok and got it done for about $30. Well, I have a really unstable mixture at idle (mostly lean) and am wondering if an exhaust leak can cause this and also the hesitation that occurs at about 1000 rpm every time I start to accelerate. The car almost dies sometimes. I can smell a very faint exhaust smell if I'm leaning over the engine with the hood up, but I can't hear any leak(s) even while driving under boost. I have: 81zxturbo with: -85300zxturbo ecu swap and mass airflow sensor and crank trigger -capped off egr and aac valves -almost all new vacumm lines -can't find any intake vacumm leaks -idle switch is AOK I'll have my new factory intake/exhaust manifold installed in a few days when it gets here, so I guess if no one replies, I should find out by then. Just trying to get a heads up so I can know where to look should the gasket not change anything. Thanks guys, I'll post if I find out what's going on. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 4, 2004 Share Posted October 4, 2004 I had a small exhaust leak when we first did the turbo swap before the head flipped out on me and had to be rebuilt. As far as I could tell, it did not affect driveability at all... only noticeable 'symptom' was the audible 'ticking' that drove me nuts for a while.... However, one thing I can think of... I dont run an O2 sensor. Now... if you're leaking exhaust, this could, I imagine, mess with the O2 sensors readings, possibly causing some small problem? Only thing I could think of. Quote Link to comment Share on other sites More sharing options...
Guest 2slo4u Posted October 4, 2004 Share Posted October 4, 2004 Yeah, I'm kind've wondering about the 02 sensor's role in this problem. Wish I could trick the computer into running ok without it. If I don't find out what's going on with this motor soon, I'm thinking of getting a brand new head from Nissan. When I checked a couple of months back, they said they could get me one, and that it wasn't rebuilt. I'm now questioning the valve job I had done and the condition of the springs and valve stems. The guy seemed to know what he was talking about until he said it didn't matter which rocker arm went where on these heads. BTW, Bastaad, which ecu setup are you running? I'm using the 85 turbo ecu and I think I have to keep the O2 sensor connected. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 4, 2004 Share Posted October 4, 2004 '83 ECU, and this car, along with two 280zx's I've owned, have always run better with the o2 sensor disconnected... dont know why, and really don't care anymore I wouldn't take the thing about the rocker arms as a sure sign your machine shop didn't know what he was doing. I've dealt with three different machine shops with my Z's and actually, 2 out of three of them said that they don't worry about marking and replacing the rockers in order. The third machine shop, said he would mark them and put them back in order, that he just felt better doing it that way, though he strongly felt that if he didn't it wouldn't cause a problem. So it seems pretty common for machine shops not to worry about that... I dont know if that would indicate that your guy didn't know what he was doing. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 4, 2004 Share Posted October 4, 2004 Over the past few weeks I have been trying to tune the car using the stock O2 sensor, a digital multimeter, and the AFM. I got the car to run around stoich during idle and cruise. The O2 voltages flipped past 0.5V every second or so and the green LED on the ECU blinked accordingly. The car ran pretty good and idled pretty good but it wasnt consistent. It seemed to run good and smooth and then the next day it would surge and miss in the 2K-3K rpm and at idle. I already checked a dozen times for intake/exhaust leaks using several well known methods and havent found any. I decided to unplug the O2 sensor about a week ago and the idle smoothed right out and the surging during cruise minimized. I did have a lack of torque in the 2K-3K rpm range so I richened up the AFM a few teeth untill I got power back in that range. Now it seems to drive better than it ever has. There is only a tiny bit of surge/miss in the 2k-3k when boost is climbing through 10psi. This is without the o2 sensor, and with the TPS connected. I can't yet comment on fuel mileage but I suspect it has suffered. One other point. About 2 minutes after a cold start, it runs like "pooh" for about 1 minute. After that it runs normal again. This was the case both with and without the O2. Bottom line, I think the tune of the EFI has a much greater effect than a small leak. However, small leaks should be fixed before they create bigger problems. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted October 4, 2004 Share Posted October 4, 2004 I've ranted about my experiences with the 02 sensor many times... good to know someone having the same experience as me (sorry Cygnus). Funny you mention about the cold start thing... I haven't ever mentioned this problem here as it didn't bother me enough to try to track down, and anyways didn't figure anyone might have a good answer for it... but anyways, my car does the same thing. When started cold, after about 30 seconds to 2 minutes, the idle will suddenly drop and get very rough. After a few minutes, right around when the temp gauge starts to register movement, the idle will suddenly jump back up and smooth out again. I dont know if this is the cause or the effect of the rough and lowered idle, but I decided to check the timing while the car warmed up from full cold, and after that first minute when the idel suddenly dropped, timing retarded from 20* to about 12-14 degrees and gets kinda erratic, then goes back up to 20* when the idle smooths out. And Cygnus, fuel mileage does suffer with the O2 disconnected... but not much. I noticed about a 2mpg average decrease on this motor. Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted October 4, 2004 Share Posted October 4, 2004 Hmmm about 2mpg less. Hmm, I could probably sell the O2 sensor to pay for the increase in fuel consumption. I only do about 3K per year on my Z. That's weird...I used to be a donating member but now I'm just a member again. Quote Link to comment Share on other sites More sharing options...
Guest 2slo4u Posted October 5, 2004 Share Posted October 5, 2004 Me too... and my stars went down. I guess after a couple of months you lose stars and donating status? 81zxturbo 95slobaru Quote Link to comment Share on other sites More sharing options...
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