Alright. I need some help.

[Soilsack]

As a disclaimer I feel it needed to be said that I have attempted searching and all it is getting me is more confusion.

I'm having some issues with a stock KA engine. Rough idle, random stalls, bad mpg, service engine light.

1998 Nissan 240SX SE with 98,000+ miles.
Stock KA24DE engine with new stock air intake, stock exhaust, new electrical system.

An explination of the issues is as follows:

1A) When at rest and I have the A/C on it idles fine. When I turn the A/C off the RPMs drop to around 250 and shudders. Sometimes it is able to fight its way back to about 750 RPMs and sometimes it stalls. If I assist with the accelerator or turn the A/C off while driving it seems to stablize.

2A) The service engine light is on and the OBDII reads that the O2 sensor left bank is reading the system is too lean.

3A) I'm only getting about 15 mpg combined. I admit I do drive it hard sometimes but I feel that it shouldn't be THAT low. It's not like I'm gunning it at every light and flooring it to 100mph.

What I am considering, looking into, and am asking about:

1B) What would be the likely cause of my issue?

2B) I am considering sea foaming the engine. Is this a good idea?

3B) I also want to do a full tune up after seafoaming. Change oil and filter, change fuel filter, change spark plugs, and possibly change the O2 sensors. Can any of this be avoided and are there any recomendations as to brands/manufacturers. Also what type of oil do you suggest for a stock KA?

4B)Are there any other things that I should look into replacing/repairing?

Please answer each concern/question by addressing the number value I have assigned (e.x. "1A) [answer here]") simply to streamline the process.

Any help or info. is appreciated.

[fliprayzin240sx]

Whats your idle at when the AC is off? Might be a dirty IACV. Seafoam and a full tune up would definitely help if it hasnt been done in a while. Double check the timing and maybe replace the o2 sensor too while your at it.

[Soilsack]

With A/C on it's around 1k, with A/C off, and the idle finally stable, it sits at about 750. The problem is, if at rest and I turn the A/C off it drops down to about 200 RPM and it surges back to about 750. It makes this cycle for about 30-40 seconds and then stabilizes. It takes less time to stabilize if I tap the accelerator but sometimes it just can't fight back and totally stalls. I'm looking at carbon buildup to be a possible issue but as I said before in searching I only get more confused. Parinoia sets in when I read so many horror stories.

[Soilsack]

Alright. So the idle is getting better at not stalling. The RPM drops to about 500 now instead of about 200 and it only does it a couple of times then stabilizes. Noticing however that when I am at low RPM as in 1st and reverse and I release the throttle and depress the clutch it does the same thing. I checked for vacuum leaks and found none. What would be more likely? IACV or Timing?

[Soilsack]

I reset the ECU and the check engine light has not come back on yet but the idle after turning the A/C off is as bad as before. Too early to tell if it had an impact on the fuel economy. Ugh. Someone has to know something about this.

[Soilsack]

Ahhh. Someone please help. This is what I've tried so far:

Cleaned the MAF.
Seafoam
Change oil and filter.
Change fuel filter
Change spark plugs and wires.
Checked the distributor and rotor. They are fine.
Checked for vacuum leaks. None found.
Changed front O2 sensor.

The car has a surging idle and horrible gas mileage. Maybe 15mpg combined on 93 octane. I need this problem fixed soon. I can't afford the gas waste.

[Soilsack]

I forgot to add. The CEL is on and is still reading a P0171 code.

[Soilsack]

Ok. Confirmed. No MAF issue, no vacuum leak, no iacv or throttle body issue, no TPS or CTS issue, no timing issue, no rotor/distributor/spark issue. The only thing I can think of now is fuel injectors. Anyone else?

[fliprayzin240sx]

Whats the code? I'm familiar with nissan codes...Did you double check the distributor timing? I still think its a dirty/bad IACV. Might wanna try taking it off and clean it out. Also, double check the EGR, might be dirty too or the the vacuum line to it might be cracked.

[Soilsack]

P0171. I just said it's not timing, it isn't IACV, and it isn't a vacuum problem, including the EGR.

[Soilsack]

Ahhh. I just realised I never updated this. Haven't been on in months and figured I should inform you what I have found so that it may further assist any future issues.

After checking, double checking, and triple checking everything including going through 2 O2 Sensors and quite a bit of brake cleaner, as well as damn near rebuilding the engine I found the issue. It was a single wire in the wiring harness. I finally realized while on the diagnostic that when I played with the O2 sensor lead from the wiring harness it would intermittently show that it is reading the A/F Ratio. so I took that nasty heat shielding off and found that one of the O2 lead wires were broken and shorting. After replacing that section of wire there was never an issue again. Hopefully this information will help someone.