Cold start rough idle/lean problem

[banipal19]

Any luck with this issue? I have same exact problem but mine happens around 2K rpms. Once the closed loop kicks in, everything is perfectly fine.

[SilverXJ]

No luck yet. I haven't worked much on it either.

I tried a stock PCM, thinking that the reprogramming of the PCM could be at fault, but its not. Ran the same with the stock unit as the modified. Which is a good thing because the company that reprogrammed it (Mike Leach) got out of it a few years back. I called them and asked if they could do an adjustment.

[SilverXJ]

Well, I couldn't find anything wrong. Maybe the PCM just doesn't like the cam 100%. Or the stock coil can't combat the increased compression. Maybe injectors aren't 100% sized.

Other than that I have what sounds as a slight miss at hot idle. I believe it is a lean miss condition. Not enough for the engine to notice, nor the RPM to change but you can hear it in the exhaust. I have noticed that if I change the outgoing MAP voltage on the Apexi that i can make the AFR a bit rich and the engine will smooth out. However, that only lasts for a 30 seconds and since the PCM is looking at the O2 sensors it will bring the AFR back down to 14.7:1.

Right now the spark plugs are one heat range colder from stock with a stock gap or .035". I am thinking of increasing the plug gap to .040" and seeing what happens.. perhaps playing with a Viper coil to replace the coil rail.

[SilverXJ]

Still dealing with this.

At first start the wideband O2 sensor shows lean, but the exhaust smells rich. However, if I add fuel with the Apexi it the idle will clean up and AFRs will go richer. The idle condition only lasts like a minute or so. I assume that is the point it starts to read the O2 sensors. Then the PCM tunes out and adjustment I did on the Apexi, so it will only work once. Maybe it is actually some sort of misfire. I don't know. All the sensors check out good. Maybe it is the cam, but I haven't heard of anyone else with this issue.

[92tank]

mine did the same thing with stock injectors really bad, even when warm. then i put in the new injectors and now it only does it when cold, but only for a short time. (92 with 24lb ford injectors)

[SilverXJ]

Im thinking of putting the stock injectors back in to see what happens. I had 28 lb/hr injectors and it had a horrible rolling idle at cold start. I currently have 26 lb/hr injectors and the fuel trim is at -13%. Maybe they are still too large and the PCM has trouble handling them? I'm still confused at the lean AFR reading and rich smelling exhaust.

What cam do you have?

[Cheromaniac]

Perhaps your wideband O2 sensor is faulty. Your fuel trim suggests that you need 24lb injectors.

[SilverXJ]

I know the fuel trims suggests that I need smaller injectors. I currently have the WJ 4.7L 4 hole injectors, which are ~25.8lb/hr @ 49psi. The problem is finding an EV6 injector that will run 24lb/hr @ 49psi. Any suggestions?

I don't think the O2 sensor is faulty as when I add fuel on the apexi it does show a richer AFR. And it works as expected all other times. It is probably right and maybe it is just so lean that it is having some sort of misfires (no engine light). Remember that they measure unburnt oxygen, not fuel.

[92tank]

i have a stock 95 cam, bored throttle body, 2000 intake mani, and um how to put it, not stock intake. i have the top half of the stock box (bottom cut just to hold the filter) turned upside down under the cowl hood.

[Muad'Dib]

The problem is finding an EV6 injector that will run 24lb/hr @ 49psi. Any suggestions?

I found tons of EV1, EV1.3 and EV6 Injectors that can flow that much at 49psi... however they all were using the EV1 style connector. Have you considered EV6 to EV1 connectors??

I was able to find BOSCH 280-155-780 (Chrysler 53031571 for 1998-03 Dodge Ram 2500, 3500 8.0L V-10) which looks like it might use the EV6 connector and flows 22.3lb/hr @ 3bar = 23.67lb/hr @ 49psi.

[SilverXJ]

I don't think those would fit

When compared to others they look short.

I have though of converting from EV1 to EV6 but I would rather not.

[Muad'Dib]

Yeah i didnt even think about the length... it may be worth it to shoot WitchHunter or MotorMan an email or phone call to see if they can provide you with some additional specs.


EV6 to EV1 connectors can be had pretty cheap... and as long as you find a EV6 Injector that uses a EV1 connector, you wont be "downgrading" per-se. Plus you can always just take the adapter out in the future if need be.. your not soldering these in.

[Cheromaniac]

I don't think the O2 sensor is faulty as when I add fuel on the apexi it does show a richer AFR. And it works as expected all other times. It is probably right and maybe it is just so lean that it is having some sort of misfires (no engine light). Remember that they measure unburnt oxygen, not fuel.

I still can't get my head around why your wideband would show lean AFRs while the exhaust smells rich. This is only happening when the engine computer's in open loop mode and your fuel trims indicate that the engine computer's pulling back fuel. This goes against your hypothesis of the engine running lean.
I suspect that the 13" of vacuum is playing tricks on your engine computer. You said you were unsure about the reading so maybe you should check it with a vacuum gauge rather than trying to extrapolate it from the MAP voltage. With your cam the vacuum should be more like 17". You might then be able to dial in the AFRs correctly using the Apexi and your existing injectors rather than swapping in smaller ones.

[SilverXJ]

I still can't get my head around why your wideband would show lean AFRs while the exhaust smells rich.

If it is having misfires, which it sounds like it does it could explain the lean AFR. An O2 sensor measures oxygen. The more oxygen it sees, the leaner it thinks the mixture is. A misfire will send unburnt oxygen out the exhaust. That is the only explanation that I could think of to explain the rich smelling exhaust yet lean AFR reading. As well as making it richer helping it smooth out.

This is only happening when the engine computer's in open loop mode and your fuel trims indicate that the engine computer's pulling back fuel. This goes against your hypothesis of the engine running lean.

I don't see how. If it is running lean enough to cause a misfire then that just contributes to the lean condition.

Maybe the PCM can't handle the -13% fuel trim properly. Then again it could be something else causing the misfire. Is the misfire caused by a lean condition, or is the misfire causing the lean condition? Maybe it doesn't like the plug or heat range, Maybe I have two stock coils that are weak, maybe the plug gap isn't where it should be, maybe it just doesn't like the plugs.

What exactly does the PCM do at the first cold start? I would assume there is something like a choke, but would it make it run leaner to get the O2 sensors and cats hot or richer to help the engine?

I suspect that the 13" of vacuum is playing tricks on your engine computer.

Perhaps, but I think something else is going on here. Maybe the misfire could be causing the lower vac number. It does have a slight pop to the exhaust even when at a hot idle. Also, changing the voltage on the MAP is the same thing as making the PCM see less or more vacuum.

You said you were unsure about the reading so maybe you should check it with a vacuum gauge rather than trying to extrapolate it from the MAP voltage.

I said i checked it with a mechanical gauge and it agreed.

With your cam the vacuum should be more like 17".

I agree. Don't know why its lower than it should be... again, could be the misfiring. And being a waste spark who know what is going on in the exhaust stroke.

You might then be able to dial in the AFRs correctly using the Apexi and your existing injectors rather than swapping in smaller ones.

Not possible. While in open loop I could dial it out at the first instance. Once I dial it out the PCM will notice the changes and just bring it back where it wants it. On next start it will be back where it is now. If this was in WOT where the PCM drops back to open loop it would work. Since the PCM is in closed loop 90% of the time in this RPM range dialing it out would only be negated by the PCM's view of the O2 sensors. It might be doable with a AEM FIC because you can offset the O2 sensors reading going into the PCM.

[Cheromaniac]

The popping out of the exhaust could be a sign of a leaking exhaust valve so it might be a good idea to check the compression on all cylinders to rule out a valve problem. It would also account for the "misfire", low vacuum, and the crazy antics of the PCM.

What exactly does the PCM do at the first cold start? I would assume there is something like a choke, but would it make it run leaner to get the O2 sensors and cats hot or richer to help the engine?

At a cold start, the PCM ignores the O2 sensor outputs (open loop) until the coolant reaches a pre-determined temp. It relies primarily on the outputs from the IAT sensor and coolant temp. sensor and sets a rich AFR as well as more advanced timing during warm-up, much like a "choke" as you mentioned. A cold engine that's running lean will have a low idle and be liable to stall on you.