I made a thread a couple months ago about this but I have realized some things are different then I originally thought. Basically I’ve got a 2000 EX I acquired recently that was parked for a few years and it’s throwing codes for misfires on all cylinders and a multiple random misfire code. It is most noticeable at idle, but with any sort of load applied (a/c, lights, p/w etc.) the misfire can nearly stop. At 1/4 to WOT it runs perfect, low rpm/load it feels like it it is just running lean/rich, but if you apply more throttle it runs much smoother. If you shut the car off and restart it runs smooth for about 10 seconds then the misfire starts again. This can be duplicated time and time again.

compression is good, 175-190 on all cylinders. New NGK coil, NGK plugs, NGK wires, Dist cap, Rotor. I have cleaned the IAC valve, replaced the PCV Valve, checked for vacuum leaks, cleaned engine/battery ground(s), performed a valve adjustment, replaced the air filter, ran injector cleaner in gas tank and a seafoam top engine cleaner. The car also has a brand new battery and alternator. I also have replaced the catalytic converter(OEM was completely clogged from prior neglect, was ran with major exhaust leak at header flange) The O2 sensor graph is a nice wave from .1 - .9 V and the STFT is usually between 3% - 11% nothing excessive. ignition timing appears correct as well. I’m beginning to think it’s an injector problem but I’m running out of direction and need some input

 

Check the EGR passageways for blockage or completely being blocked up with carbon.

 

This doesn’t have an EGR

 

Ok, last time valves have been adjusted?

 

In my first post everything I have already done was listed. The valves have just been adjusted

 

Last time I struggled with random misfires, it was the thermostat. The existing one was made with a piece of rubber on it to help it seal. But the rubber had gotten old, folded over, and was preventing it from sealing. So it never heated up, except under continued WOT. Ran like crap until you really thrashed it. Not saying this is your issue, just throwing it out there.

 

Thanks for the tip, I will keep it in mind. My car is heating up and maintaining temperature appropriately and it doesn’t seem to be connected to engine temperature as it can do it stone cold or after running for hours. The one thing that makes me feel like it’s a huge clue is the shutting off and restarting part. It can be cold, warm, or hot, if it’s misfiring at idle and I shut it off and restart, I get about 10 seconds of smooth running….. I feel like that’s a major clue but the only thing it points me to is a fuel issue. it just seems weird how it runs so great at larger throttle percentages

 

Things you might try that won't cost you too much are: 1) a new fuel filter. 2) Something available online called a "FUEL PRESSURE GAUGE ADAPTER BANJO BOLT FITTING 12MM BOLT KIT 1/8" NPT FILTER". You use it in place of the banjo bolt on your fuel filter and you can screw a 1/8 npt pressure gauge into it. Useful for checking fuel pressure. 3) Remove and clean fuel injectors.

 

Those are exactly the next steps I was heading to, fuel filter and then physically removing and cleaning/or replacing injectors. I just needed to take a break for a minute and put it out there to see if I’m missing something that should be obvious aside from what I’ve tried. I have thought about the igniter but I don’t see how it would relate to my symptoms. Although the ignition system was over all in very poor condition when I acquired it, it wouldn’t even run. I still feel that this is a fuel issue. What’s the most odd though is that I am getting black smoke (Fuel) when I free rev or accelerate at WOT and I feel that if It was a clogged fuel filter or injector that I would be starved for fuel not rich like it is running…

 

Thoughts on a faulty FPR? Would one that’s stuck closed cause a rich condition and misfiring at idle and low throttle inputs? Doesn’t really explain the smooth running in open loop…. You can watch on a scan tool on startup and almost the instant the ECU switches into Closed Loop operation the misfires begin……….

 

which IAC valve do you have?

 

It is a Denso 2-wire unit…. I was very suspicious of it initially being that the misfiring is tied to fast idle coming down but it isn’t throwing a code for it and the idle systems all seem to work correctly. The idle isn’t too high for too long or too low, and it’s doing a good job at regulating for loads etc. it just is so weird how I can shut the car off and re start it and have no misfiring at all until the idle comes down and the Ecu returns to closed loop after running for 10 seconds or so…

 

Ok, after killing my head over this for about a month now I decided to check something. Watching the scan tool reading of the ECU switching from Closed Loop to Open Loop. BINGO. If I slowly accelerate from say 2K rpms in 2nd gear and let the rpms get high 5K+ it gets very coggy feeling, as soon as just a tad more throttle is applied and the ECU switches to Open loop, it’s like a switch is flipped and it’s running like a top. The same applies on deceleration, there is a car shaking change in smoothness when the ecu is coming out of closed loop and smoothing out in open loop on high rpm deceleration. This is also the same thing that is occurring upon startup. So the fuel and ignition systems are fine but a sensor is not giving accurate feedback in closed loop.

now the question is what sensors are being used to calculate in closed loop? I know oxygen sensor, but are there others?

 

The primary O2 sensor is your main issue... however, your IAT and ECT sensors could also be contributing to the problem. Back to the O2 sensor: an old tired/worn out sensor could send incorrect values to the ECU and thus cause huge swings in the level of increase/decrease in fuel trim. Also, an exhaust leak up stream of the O2 sensor could be altering the readings of the sensor and causing your issue, so look for black chalk or burns on the exhaust system or head/block. Repairing these may solve your issue... and if there is no leak, a quality replacement O2 sensor should get you back to normal. I only recommend either an OE Honda one or one from NTK.

 

You could have an injector hanging open slightly or working intermittently. However that’s not likely but possible.

I would change the fuel filter and the upstream O2 sensor and make sure you don’t have any exhaust leaks. If your car has the manifold catalytic converter, those like to crack right down the middle and that can cause very inaccurate O2 readings.

 

whats your vacuum at idle?, pull off the vac hose at the fuel pressure regulator, if gas come out of the fpr, theres your problem

 

I am going to replace the O2 sensor asap. I have no exhaust leaks, but when I got the car there was a huge exhaust leak at the collector just in front of the cat where one of ears broke off of the flange on the header and it was apparently drove this way for long enough to completely clog and destroy the cat. That has since been repaired and there are no leaks now anywhere in the system, so a new O2 should be able to work properly.

yes I’m going to do a fuel filter and an O2. I am leak free and this is a Y8 with a standard exhaust manifold and under car cat. And I still haven’t completely ruled a sticky/dirty/failing injector(s) out yet… I’m saving that for last

That was something I originally checked and had no fuel in my vacuum hose, but since the car sat parked for a few years I was thinking about the possibility of it being seized and not opening upon vacuum thus increasing fuel pressure to max at idle and therefore causing my rich idle and low throttle inputs, when the vacuum would normally open the valve and reduce fuel pressure.

 

Well, today I replaced the upstream oxygen sensor with a brand new NTK sensor and replaced the fuel filter. No change whatsoever.

Again, shut the car off and restart it, and it idles smooth as glass, until the ecu switches into closed loop.

My exhaust has absolutely NO leaks, compression is good. This is purely something controlling the fuel in Closed Loop but what would have such an impact if not the oxygen sensor???

IAT and ECT sensors both read correct numbers on the scan tool.. what in the **** is going on here….

 

Check your TPS sensor calibration! make sure the TPS is reading correct at WOT and closed! very easy to do.

 

I have been thinking about the TPS for weeks, but when I compare it with my two other Honda’s, (99’ Si and 00’ CR-V) the values on the scan tool read almost identical on all 3 vehicles. It shows the throttle position as 9.2% at idle and around 93% at WOT. I understand that’s not a perfect calibration but It’s been my understanding that’s the TPS shouldn’t read a true 0% at idle because the butterfly is open past 0% to keep from completely choking. Also the fact that the other two run great at similar values. I plan to calibrate the TPS especially if this seems like it’s wrong. It’s been something I meant to try next. Is it OK to use the scan tool for TPS calibration or is this method completely wrong and irrelevant, and I should just be looking at voltages ?

also I can say that this problem Is greatly exacerbated when the engine is run at high rpms and very low throttle inputs. The ecu pulls up to and over -30% STFT at times and STILL chokes on too much fuel under the right conditions. But at idle will always sit around +5%/-5% … but this thing is chugging fuel. Lots of Black smoke, even visible from inside the car when I look behind me, and I can smell fuel anytime I drive the car. I know it’s way over fueling.

 

Set the TPS with a volt meter... key on, engine off. Voltage should be .490-.500 volts... and between 4.20 and 4.60 volts at WOT. Have you checked your fuel pressure ? Also, what is your output voltage from the alternator at idle and cruising above 2500 RPM ?

 

I will set the TPS with a voltmeter this weekend. I ordered a fuel pressure gauge yesterday to install on top of the filter. I will report back when I get readings, and I agree that it seemed it could be fuel pressure related, as I’m aware of how sensitive these are to fuel pressure, the only thing that threw me off of that diagnosis was the fact it’s running good in open loop…

I just recently replaced the alternator because when I got the car and initially got it running I noticed it was overcharging, the headlights were pulsing and it was reading to 15.5V and even spiked higher then that a couple times.. I immediately stopped running it and replaced the alternator with a used low mileage OE alternator and the voltage has been a steady 14.2V since then, maybe slightly lower to 13.8V at idle with accessories on, but honestly, normal. I had wondered if some damage had been done to the ECU or other electrical components from the overcharging, since I have no idea how long it was ran like that before I got the car, but everything seems fine other then this rich condition misfire..

Hopefully the fuel pressure readings will lead me into a direction, I am about tired of fooling with it. I just want this sorted so I can move further with this project

 

Okay so it took me a few days to get back at it for a little bit, but I was able to perform a few diagnostics.

fuel pressure reads right at 37psi at idle with the vacuum hose connected and does rise to about 44psi with the hose disconnected. These readings are right within spec and when I watch fuel trims with the scan tool I even see it trying to correct by pulling fuel when the hose is disconnected. The fuel pressure is 100% correct and I know the FPR is working. I am ruling out fuel supply.

Also I tested the TPS with my voltmeter and the voltage was exactly fluctuating 0.49-.50V at closed throttle and 4.39V at WOT, directly within specs..

Every sensor that appears to be responsible for fuel is testing perfect, yet I’m still getting excessive fuel, black smoke, and intermittent misfiring. I have listened to all my injectors. They are all clicking and sound even.

remember, at higher throttle positions it is running great.

 

 

the engine needs a balance of fuel and air to run correctly. since you are spewing black smoke and all the fuel components are in good shape, its time to look at the air side of the equation. Something is clogged up/not working as it should to allow the air into the engine. To test this theory, find any and all vacuum hoses at the intake manifold (there aint that many) unplug them and go around the block. If it runs even remotely better, then its an air problem. As stated before, you cleaned the IAC till clogged? Just because yiou dont have a cel code for it, doesnt mean its now working properly. I chased my tail for a month before I found that a janky iac with no codes caused exactly your problem