FireDog

I need some help yall.

I have a very random misfire (I think) but it's not throwing a code so I can't isolate it. It happens randomly and at all driving situations from idle to spritted, I've stayed out if vtec though. Also it may happen just once or keep missing for a second. Sometimes while driving the whole car will jerk back like you hit something or like the rear end was missing a tooth but neither are the case. Other times the power goes down and it makes a rapid low sputtering noise for a few seconds, doesn't drop rpm, then regains power and sounds normal. Either can happen from idle, acceleration or cruising. I can't figure it out. The car is a 00 AP1 with about 140,000 miles on it.

(What I've done)
No vacuum leaks.
New spark plugs.
New coils.
Cleaned the iacv the right way.
New bilman TCT.
MAP wack and cleaned the channel.
Cleaned throttle plate and plenum.
Fuel antifreeze in case it was bad gas.
And less than a year ago I had the injectors professionally cleaned/rebuilt.

Anyone delt with this before? Any ideas?

Slowcrash_101

Check your valve clearance, how does the car run when you plug the vacuum line from the FPR? Do you have an aftermarket intake? Could also be a problem with the fuel pump, is your O2 sensor still the original? Might be a good idea to look at fuel trims, and O2 sensor voltage. You may also have an exhaust leak somewhere, maybe the ex. manifold.

FireDog

I am planning on doing a valve adjustment tomorrow.

I pinched off the vacuum line the the regulator and it made no difference at idle or revving it by hand. What does that mean?

I do have an intake. Just a basic short tube intake, I've had it for year and its never been an issue before but who knows.

Yes, it is the original O2 Sensors.

How do you check fuel trim? I have never done this before.

I think the car might have a small exhaust leak, I can hear it now and then but again its never been an issue before. This misfire is a new thing.

I might also add that I am in Houston where its crazy humid and stupid hot all the time. Is there any way that this may all be heat soak issues?

Slowcrash_101

Pinching the regulator boosts fuel pressure by a few psi. Try driving around like that you'll feel the engine a bit peppier. At 140k that O2 sensor might not be as sharp as it used to. If it can't give the ECU the right signal it may affect power and fuel mileage. Smell your oil, does it smell a good bit like gasoline? Aftermarket intakes are prone to heat soak, gold wrapping the part in front of the radiator seems to help, and also gold wrapping the intake box if you have one.

If you have access to a scanner, you can read the fuel trims, you can see if the computer is compensating for a rich or lean condition.

If you have an exhaust leak before the primary O2 sensor it can also affect power because it may be reading more oxygen than there actually is and leaning out your mixture.

FireDog

This sounds like sound advise thank you.

I'm going to drive it with the line pinched and without it pinched, tonight while its cooler then tomorrow in the heat and see what happens. I figure it should help show if its heat soak and also low pis in the fuel line.

I have a OBD2, its not pro grade but it does show live readings. What should a good fuel trim read at?

Also the exhaust leak is at the joint that goes over the axle so that should be well behind the O2 sensor.

s2000Junky

Sounds like oil consumption issues as one possibility. If you have significant enough oil getting past the rings or valve guides intermittently, it will have very similar symptoms to what you describe. This is always on the radar with an ap1, esp with 140k miles on one. So how much oil do you burn every 1000 miles, do you know?

s2000Junky

Well you have a long term trim and a short term. These have a general swath range that they will fluctuate between depending on many factors, ambient temp as well as under hood temp/heat soaking or not and then your fuel pressure and primary 02 sensor. As heat rises they should show - readings and cooler driving they will generally show +. They have a 0 target the ecu works from and should straddle this within +/- up to 10 typically unless there is something more severe happening such as a lot of heat soak and or fuel pressure issue. These trims run off of a 02 feedback and the set parameters of the ecu target.

Just as an example, with ambient and IAT nominal, I see negative long and short term fuel trims when I turn up the fuel pressure, which is the ECU way of dealing with the added fuel to run its centered target afr in closed loop. So the opposite effect of low fuel pressure would show + fuel trim numbers. All this said, if your seeing consistently -/+ trims in the mid teens or higher, you very well may have a fuel supply or pressure issue, or some other issue related to the primary 02 sensor. But these are just educated guesses and clues to deduct from there. Really would be helpful to trigger a cel. And you should eventually if this is a severe enough issue.

FireDog

Ok just did my night drive.

Started the drive without the pinched hose and it went well. After I had been driving a while it might have had a small miss once but I am not even sure about that. When I think about it I think it always gets worse the longer I drive.

I pinched the hose and drove home and sure enough I felt more power but a strange thing happened. I wanted to test acceleration but there was no light so I slowed to about 22 and accelerated out of 3rd, too slow for that gear. When I did it missed. I did it a few more time and each time it did the same thing. When I was in too high a gear under about half throttle it would misfire.

This is the first time I got it to miss repeatedly. Would that tell anything about why its happening?

FireDog

Hi Junky thanks for the reply.

Speaking about the oil burning. I do not know how much oil I loose per 1000, but if it was a good bit shouldn't it be having blue smoke exhaust or are we talking about less oil than that? If it was from oil, would it be cylinder wall scoring, cracked rings or something less major? IE. is this a rebuild situation?
Its been about a year since I have done a compression test so I'll do one tomorrow and see what it turns up.

Also, thanks for the advice on the fuel trim I will run it tomorrow and see what it turns up.

FireDog

I had a local guy tell me that he had the exact same issues with his S and had no luck figuring it out. He tried everything I have listed and then some and nothing worked but he unintentionally fixed the issue when he installed a test pipe. He said he was just installing it as a mod but it fixed the misfire so he figures it was a clogged cat.

Has anyone had issues with clogged cats, and how do you check for that?

I have had a few times in the past where I have run rich to the point that I got a cel about it and I know that can burn up a cat so its not out of the question.