420R Misfire

[savagete2860]

Last time I had an issue with my car with sticking RBTBs this forum helped me tremendously. I have a new issue I am hoping I can get some help and advice on.

 

Pretty much the same day I got the roller barrels to stop sticking, I noticed on cold starts or for a little on hot starts (in open loop) the car would idle pretty low - bouncing from 5xx-700 RPM. Once in closed loop, things would behave as far as idle RPM is concerned. The bigger issue however was trying to press the gas off idle would bog the car down before the RPMs start climbing. Drivability was pretty poor and would lead to stalling especially on hills. I ended up setting the TPS voltage a little lower than normal which bumped the idle enough you can fairly easily drive around the issue. It would still miss though, and you can hear it pop from the intake occasionally.

 

I ended up ordering an Easimap cable to properly set the TPS voltage. It seemed like even at 4.62v at the middle pin it wasn't running right so I figured the ECU might see a different value so to be sure I wanted to check it with Easimap. Turns out that was correct - there was 0.03-0.04v difference between what the ECU would see and what my multimeter on the middle TPS pin would see (Easimap values were lower). I adjusted it to exactly 4.62v seen by Easimap and wanted to test if it would still have a low idle and misfire. Same deal unfortunately.

 

With the TPS set correctly now, I fired up the car and while idling, I disconnected the cylinder #1 coil. It didn't change the idle at all. I moved the COP and the plug from cylinder #1 to #2, but the problem was still at #1. All the other cylinders seems to cause an RPM drop when disconnected aside from #1.

 

Of note, I pulled all of the plugs and they were pretty black. Short term fuel trim was pulling 20% fuel as well (not sure if that's because the TPS was misadjusted for a while and long term fuel is wrong now or what)

 

I read the "coil driver" on the ECU can go bad or the loom wiring to the COP. I guess I am wondering if there are any good ways I can test that. Also is there a way I can rule out any other issues, like the fuel injector on #1?

 

Since I crossed over the 500 mile break in period and I have not be shy with the WOT. It doesn't seem to misfire with load. Not sure if that is another clue - or maybe I have been driving around a 3 cylinder Caterham and I just assume it feels normal.

 

Any thoughts?

[Vovchandr]

Swapping injectors around would isolate it to either being an injector or ECU/electrical not sending the signal to fire. Id proceed with a swap. 

 

You can also try to hear it firing with a stephoscope if that's easier or available. 

[KnifeySpoony]

Test continuity of plug wires with a meter. I had one go bad. 

[savagete2860]

I will test swapping the fuel injector and with continuity check the plug wires - those seems like solid first steps and if those are both okay I would guess I'd be looking at the ECU itself.

 

I am a little bit of a smooth brain though so I need some help with the continuity testing. I understand I will need one probe at the plug that connects to the coil, but where would I put the other?

 

EDIT: I checked just the sub-loom and it was fine on #1. Not sure if there is a way to test it further back or not.

Edited September 4 by savagete2860

[savagete2860]

Moving the injector didn't do anything. I continuity tested the sub loom wires for the coils and the injectors and they all are good.

 

Here is a video I took yesterday when I decided #1 was an issue (its in open loop during this video)

https://imgur.com/a/iAGt4mH

 

This is a video of the car when it just got into close looped from a few days ago. The TPS at this point was still lower to help with the misfire issue. Hopefully you can hear the miss but more notable is the temperature on cylinder #1 being much lower than the others.

https://imgur.com/a/RKxOROV

 

And here is a video of the idle tip in not cooperating - each throttle application is about the same. THe one that revs high must have been a little slower but it seems inconsistent.

https://imgur.com/a/CBKiJ3A

 

Based on those videos do I have a legitimate issue? I am starting to wonder if I am just going crazy. Is it possible this is just normal and my fuel trims are out of whack. Is it normal with RBTBs that opening quickly from a low RPM just adds more air than the system can compensate for?

 

If I am not crazy and this doesn't seem right, what are the potential issues?

- Wiring from the sub loom to the ecu?

- ECU itself?

- Vacuum leak somewhere?

- Could long term fuel trims just be way off from the TPS voltage being wrong for so long? Does the car use LTFT in open loop? Only wondering this since STFT according to Easimap were pulling 20% fuel meaning LTFTs were way off what the car wants.

Edited September 4 by savagete2860

[KnifeySpoony]

Hmmm no sound on videos for me. 

[MV8]

Looks like a power or ecu connectivity issue with the #1 COP connector or wiring.

[savagete2860]

There should be sound on the previous videos, they're muted by default you to have to un-mute them on the bottom right.

 

I pulled off the roller barrels again in hopes I would find a vacuum leak from the intake seals. Sure enough one of the gaskets was stretched and no longer fit in the groove. I didn't have a replacement so i just cut some length off and put RTV on and assembled the throttle bodies again. I took the car to an autocross yesterday for LOG44 (I am a local) and the car was misfiring really bad trying to free rev the car to launch it. I thought that means the seal wasn't the issue but I checked with Easimap today and the TPS migrated to 4.65v! I set it back to 4.62v (it shifted to 4.61v though) and I drove it around today and it seems fine now. STFT still seem out of whack and cold start and idle isnt great, but the bulk of the bogging seems to be fixed. I wont call it 100% until I drive around some more but its headed in the right direction.

 

I made a video that hopefully gives some insight to the issue or non-issue.

 

STFT is the % value above RPM in the videos.

 

Note the lam value on the bottom left (actual value) and the one to the right of it (lam target). These never match at idle even in closed loop - is that normal?

 

Also looking for protips on how to make the tps not migrate from where its set.

Edited September 8 by savagete2860

[KnifeySpoony]

I can't help with your low idle. Mine idles at almost 1100rpm. If your motor is running fine and pulling at higher rpms, then I think your throttle blip thing is just a brief lean stumble and not a misfire. If i do a big but super brief throttle stab at idle, mine will stumble too. 

[savagete2860]

18 minutes ago, KnifeySpoony said:

I can't help with your low idle. Mine idles at almost 1100rpm. If your motor is running fine and pulling at higher rpms, then I think your throttle blip thing is just a brief lean stumble and not a misfire. If i do a big but super brief throttle stab at idle, mine will stumble too. 

That's good to know you also can cause a small stumble from idle with fast throttle changes. If that is normal I can live with it - since my last adjustment it has been way easier to drive than before where it would actually pop from the intake try to stall the car. The car feels totally normal everywhere aside from off idle - and now it feels pretty close to normal.

 

If you remember the next time you have Easimap hooked up to you car would you be able to see if you also have similar STFT at idle when hot and is your lambda reading and targets are different? Those might also be "normal" but I haven't been able to find anyone else discussing it.

[slowdude]

45 minutes ago, savagete2860 said:

That's good to know you also can cause a small stumble from idle with fast throttle changes. If that is normal I can live with it - since my last adjustment it has been way easier to drive than before where it would actually pop from the intake try to stall the car. The car feels totally normal everywhere aside from off idle - and now it feels pretty close to normal.

 

If you remember the next time you have Easimap hooked up to you car would you be able to see if you also have similar STFT at idle when hot and is your lambda reading and targets are different? Those might also be "normal" but I haven't been able to find anyone else discussing it.

Came here to mention lambda. It may not know how to run open loop which is why its struggling to give air/fuel.

 

the throttle position voltage doesnt seem like too much of an issue. It may be indicative of a ground issue.

[savagete2860]

I was driving around today and noticed the car seems to not have the same hesitation while in open loop. Took a video of my thoughts. Maybe it would help give clues? I am really suspecting the 02 sensor right now. Is there a "better" version of the OE one available int he states? I think I want to try a replacement.

 

 

To drive around the stumble, I have been doing a blip (on then completely off the pedal) of throttle followed quickly by how much throttle I actually want. If i didn't do this, the car would stall with load, especially on hills.

 

Seeing how the car revs with and without the -20% STFT and it makes sense why I had to drive it like that. I still drive the car like that. I mentioned in the video I thought the rbtb gasket was the main issue, but I'm really not so sure. I think the TPS migrating might have been more the issue. The car does seem to do better at a slightly lower voltage like 4.61 vs 4.62 and the car starts to get worst at 4.63 (this voltage actually cause the car to die on its on on a hot start) - all of those are at throttle site 0.0 and my understanding is anything under 0.4 is considered "idle" by the ecu.

[slowdude]

will need to put a resistor in line to ecu to smooth signal? If its the TPS voltage you think, resistor could smooth the noise out on the signal.

[savagete2860]

11 hours ago, slowdude said:

will need to put a resistor in line to ecu to smooth signal? If its the TPS voltage you think, resistor could smooth the noise out on the signal.

I don[t think its related to TPS voltage. For some reason at idle the car cant hit the desired lambda target. I ordered a new O2 sensor Bosch Universal Oxygen Sensor | BOS 15727 from here: https://www.oreillyauto.com/detail/c/universal/bosch-universal-oxygen-sensor/b02u/15727?q=BOS+15727 and an supposed to get it sometime today.

 

I am hoping the current O2 sensor just doesn't read properly and that is causing the issue. If nothing else the O2 sensor will at least be ruled out.

 

My kit was purchased from Caterham in 2024, the same year they switched over to the 9M4 from the 9A4. Rocky Mountain Caterham is going to be sending me out a new 9M4 ECU with the RBTB tune eventually to test with to check that off the list too.

[slowdude]

Ya TPS voltage doesnt sound right to me either. AFR is more likely culprit, especially with your open loop comment.

[sf4018]

I've a had an engine loom wire that fed to a coil fray and fail due to rubbing of the harness on the engine, it was really hard to find since it was intermittent and also underneath the loom. I would verify the loom is in good shape physically and electrically between the coil and ECU. Maybe you've done that already.

[savagete2860]

So I ordered an O2 sensor but the connector is different. I can't seem to find a good replacement stateside. Anyone have any ideas?

 

Also looking for where to buy replacement spark plugs - any suggestions?

 

I took the car to its first track day yesterday (engine cover removed). The car had big time misfires in the 2nd session and I had to end a few laps early. The cars power would randomly totally get sucked away. I attempted a 3rd session, but the engine would sap all power pretty frequently. I was able to limp it home but even on the highway it was ocassionally dying then randomly would pick up again. Plugs seemed okay - no oil or fuel on them, subloom looks visually fine. Wondering if this is unrelated to my other issues, like maybe the TPS couldn't take the heat of a track day. It could also be the originally issue getting worse and this is now how it presents. I will dig into the car a bit more this weekend and see if I can find anything.

Edited September 19 by savagete2860

[wemtd]

Depending on the connector you can re-pin it. My Haltech ecu uses a Deutsch connector for O2 sensor. Sensor is standard Bosch unit. I replaced stock pins with a Deutsch connector. 

[savagete2860]

34 minutes ago, wemtd said:

Depending on the connector you can re-pin it. My Haltech ecu uses a Deutsch connector for O2 sensor. Sensor is standard Bosch unit. I replaced stock pins with a Deutsch connector. 

Good to know. I bought one of these: https://www.boschautoparts.com/p/universal-oxygen-o2-sensors-15727- so its ready to throw the included connector or another one on. I would just need to change the plug coming off the car I guess. 

 

The standard connector has extremely long pins, not sure what they are called or how to properly match them, otherwise ordering matching pins and a connector would be just as easy as converting to something like a duetsch connector. Either way one of those options might be the most economical considering the current situation with tariffs. 

[savagete2860]

I was watching back some of the video from my track day and notices the coolant temp gauge was on the fritz sometimes. Guessing this is why I was misfiring at the track? Made a quick video zoomed in on the gauge - hard to see, but it does show something weird going on.

 

 

edit: Never mind - didn’t realize the temp sender for the gauge was different than what the ecu sees. That looks like it was just from a loose connector.

Edited September 20 by savagete2860