SR20DET runs lean with Air Filter, Runs Rich Without it.

[gmorcos37]

Hello,

Having some idling issues here. It is an SR20DET Redtop. I am having a rough idle for some reason. I noticed that when my air filter is on the intake, the car runs pretty lean like about 16-18 on idle. Then when I take off the air filter, the car still starts to richen out and fluctuates from 12-14.7 on idle. The motor is also having a rough idle/misfire. Vacuum is good at 20 inHg and compression is pretty decent at 150 across the board. Please Help! Any suggestions will be appreciated!

Have checked:
-MAF: cleaned and checked idle voltages as well as linear increase with increase of engine speed- all ok
-Spark: spark plugs are new, after a while, they became white (indicates running lean), checked spark and all firing (not sure if weak?)
-Fuel pressure: good
-Fuel filter: New OEM 300ZX filter
-Vacuum: 20inhg at idle
-Compression: 150psi on all 4

Thanks!

[Edwin562]

what kind of filter are you using?

[gmorcos37]

It is some no name, but I cleaned it really well and it was running fine with it earlier. Not too sure what has changed.

[Kingtal0n]

The reason the air filter seems to change the air/fuel is because when the air filter is NOT installed, minute fluctuations in airflow across your hotwire cause subtle rich conditions. You can try blowing on the maf with the air filter off and watch your air/fuel go rich if you want.

We need more info. What makes you think there is a problem. What is the engine doing/not doing. and when did the problem start.

You said rough idle so I will start here. If compression is good, install brand new plugs, make sure your gasoline is fresh and clean, checked your fuel pressure, has stock injectors/ecu/maf, and is a recent problem, I would say boost leak test is the next step, and before you do that, try unplugging your O2 sensor to see if that clean up the idle.

Also pick up a spare maf hotwire from a SOHC KA. grab a few if you can, they are easy to find usually and cheap. You will thank yourself later when you have spares.

[gmorcos37]

The reason the air filter seems to change the air/fuel is because when the air filter is NOT installed, minute fluctuations in airflow across your hotwire cause subtle rich conditions. You can try blowing on the maf with the air filter off and watch your air/fuel go rich if you want.

We need more info. What makes you think there is a problem. What is the engine doing/not doing. and when did the problem start.

You said rough idle so I will start here. If compression is good, install brand new plugs, make sure your gasoline is fresh and clean, checked your fuel pressure, has stock injectors/ecu/maf, and is a recent problem, I would say boost leak test is the next step, and before you do that, try unplugging your O2 sensor to see if that clean up the idle.

Also pick up a spare maf hotwire from a SOHC KA. grab a few if you can, they are easy to find usually and cheap. You will thank yourself later when you have spares.

Thanks for the help. I appreciate it. I understand how removing the filter can effect AFRs but it was just strange to me that without it causes it to run rich. I feel like it should have been the opposite.

As far as the problem, the motor is running lean and misfiring. If the filter is off, it runs rich but still misfires. I can feel it do this inside the car. Also if I rev the car a bit from the motor itself, I can see the motor hesitate and misfire randomly.

I did install new plugs and the problem persisted without any changes. I checked all the plugs again and they all look IDENTICAL with the same whitish color on the bottom. This leads me to think it's random misfire and not focused on one cylinder.

Fuel pressure was good when I checked it even before the new fuel filter.

ECU, MAF, and injectors are all stock.

Boost leak test is a good idea, I will try that next as well and unplugging the O2 sensor and see what happens.

I was going to buy a new MAF but everything checked out with voltage. But I will see if I can find some of those hotwires.

Thanks again for the help. I will try out these suggestions and get back here with my results.

[Kingtal0n]

if the engine turns rich, and still misfires, you have an ignition problem.

coils
plugs
grounds
wiring

possible reason it is fuel related: wideband is wrong

it is normal to see white on healthy engines that do not misfire even with 3-5 years on the plugs.

a boost leak will richen the mixture. A vacuum leak will lean it out. check for vacuum leaks after the maf, at the tube on the compressor inlet.

[gmorcos37]

if the engine turns rich, and still misfires, you have an ignition problem.

coils
plugs
grounds
wiring

possible reason it is fuel related: wideband is wrong

it is normal to see white on healthy engines that do not misfire even with 3-5 years on the plugs.

a boost leak will richen the mixture. A vacuum leak will lean it out. check for vacuum leaks after the maf, at the tube on the compressor inlet.

I pulled the plugs and coils and tested spark and they all seemed to fire ok, I do not a spark/ignition tester tool but the sparks looked pretty consistent across all plugs. What is the best way to test the coil packs?

If my Vacuum is pretty stable at 20 inHg, can I rule that out or is a vacuum leak still a possibility?

[gmorcos37]

if the engine turns rich, and still misfires, you have an ignition problem.

coils
plugs
grounds
wiring

possible reason it is fuel related: wideband is wrong

it is normal to see white on healthy engines that do not misfire even with 3-5 years on the plugs.

a boost leak will richen the mixture. A vacuum leak will lean it out. check for vacuum leaks after the maf, at the tube on the compressor inlet.

Come to think of it, I remember one of the coil packs being a bit rough on the plastic below the plug wire. I will have to individually inspect those again. It might be some arcing thats happening.

[cotbu]

If the spark plugs fire, I usually deem coil packs good, as long as there are no cracks on the body or burn marks on the top, or melted anywhere Etc. You can ohm them and you can read the temperatures as well. The temps should be consistent across.

Your idle plays a large part in vacuum we don't know your idle. But 850 rpm and 20inhg is good. A vacuum leak in the intercooler piping might not register on the boost gauge installed after the throttle body, while idling, unless its huge.
Always pull codes and boost leak test as part of maintenance, then when troubleshooting it becomes second nature.

Sent from my Highly Tuned Galaxy S3.4!!!

[gmorcos37]

This is what I meant by it being rough. Looks like these 2 are melted a bit. Could this be a cause? Did the ohm test and got 1ohms on each coil when testing terminals 1 and 2. I also unplugged the O2 sensor and it made no difference what so ever.

72313

[cotbu]

They don't look bad to me, usually they have cracks on the body and spark will barely make it to the plug. If you swap them around and the problem follows the coil packs? Replace them.

Remove the boot examine that portion and check the spring/coil piece.

[gmorcos37]

What is supposed to happen if you unplug the 02 sensor? Should it run rich/lean or no change under idle?

[gmorcos37]

I started the car up today and noticed that the AFRs are pretty good on cold start but still runs a bit rough. I did notice that car runs pretty lean after its warm though.

[cotbu]

If you unplugged the 02 sensor before closed loop occurs, you'll be rich after 80c probably lean. Then the ecu would use the long term and short term fuel trims at idle, so it'll probably end up on the rich side 13.5_14.0. Cruise would probably be around 15.2

You wouldn't be absurdly lean like 17,18 that would mean you tune is way off.

I tune with 02 feed back off to get a 14.5afr once the o2 is tuned back on the ecu will try to keep a 14.7afr so I'll watch the wideband for 5min going from 14.2 15.2 then it will settle on 14.7 like the wideband froze in nistune the 02 would be going crazy.

Sent from my Highly Tuned Galaxy S3.4!!!

[gmorcos37]

Hmm ya not sure then. Like I said a bit earlier, it runs ok when cold but still has rough idle, but then after it heats up, it starts to run lean and has rough idle.

[cotbu]

Do a test for me. Warm the car up, let it reach 80c at least once. leave the filter on, disconnect the mafs and disconnect the 02 sensor. what are the afr's? check and clear codes.

[gmorcos37]

Ok I'll do that. I don't have a aftermarket coolant gauge so what would you say 80c is on the stock one? About halfway or so?

[cotbu]

about 2-5 minutes at the normal range on the oem gauge

[gmorcos37]

Alright so I warmed up the car for about 5 min and pulled the maf and 02 sensor. The afrs was about 9.8-10.0. Checked the codes and got code 12.

[cotbu]

if it didn't lean out your set up is functioning normally, and if you connect the mafs it should run normally rich like something 14.0-13.5 area. If it doesn't set tps to .45v(closed) set/verify base idle and timing. leave the 02 disconnected for now. The only other sensor that would lean you is the coolant temp sensor. Did you perform a vacuum/boost leak test?
If you are not using a smoker to test, separate the intercooler piping from the engine by capping of hoses.This way you can say for sure it's not the pipework or intercooler. You may find that its the, intake manifold gasket, throttle body gasket, iacv etc.

[gmorcos37]

Thanks for the help, I will get to all these tests as soon as I can.

[gmorcos37]

So the car seems to idle better when its cold then it starts to get rough and run lean once it's warmed up.Checked tps and it is at .48 closed and 4.14 at wot. I checked resistance on coolant temp sensor and the numbers matched up to the fsm when cold and warm. I tried finding vacuum leaks using propane but didn't get any idle jumps. Haven't got to boost leak test as I need to make a tester.

[gmorcos37]

I noticed something else now. If the car is idling at say 13.5-14.0 AFR and I pump the brakes rapidly, I can get the afrs to move to 18-19. I can also see my vacuum moving as well. I know the brakes are powered by engine vacuum but I am not sure if the afrs should be effected that much by pumping the brakes.

[cotbu]

You have to get that boost leak tester built, even if you fix your issue you'll want to have this tool for a turbo car always. Even if you go from a maf to a map setup.

Usually the afr will lean 1 or 1.5 afr points but come right back, I have never tried multiple pumping to try an lean out the afr. Check the check valve for the brake booster and lines, and check booster. If there's a leak after the throttle body, you might be able to hear it with the system filled with compressed air.

Also have you looked at the ecu to see if anything is burnt, broken or a daughter board installed.

Sent from my Highly Tuned Galaxy S3.4!!!

[do it sideways]

Bringing a old thread back from the dead, my sr20 is doing the samething. I have changed about everything MAf, injectors, plugs, fuel pump ad filter, ect. Runs great when cold as soon as warms up idles rough and runs lean only at idle. Any ideas or info would be great thanks!

[Kingtal0n]

try unplugging the O2 sensor