420R Misfire

[savagete2860]

So even though the track day engine power cut was extreme, the car drove totally fine the following day. I swapped out the plugs which were pretty black. Hoping that was the main cause but I guess I won't know until my next track day.

 

I also have officially ruled out the ECU - tested out a brand new unit with the RBTB tune and same deal. Off idle bogs, stalling and poor idling on cold start - everything was identical on the new loaner ECU. 

 

So now what? Well, I tried something silly. Adjusting the idle stop screw so the barrels are open more than 6mm. I opened it to an random size that ended up measuring 7.5 mm and then adjusting the TPS. Results? Car fired up and idled great cold. The idle bogging was also fixed. I could now see instead of pull 20% fuel, it was adding 6% fuel. So I tweaked the opening some more and landed at 7mm. This makes the car drive like I feel it should. Starts up right to 1000RPM cold or hot and I can actually stop on a hill without having to rev the car a few times before touching the clutch. STFT is not anywhere near 20% swings. I know Caterham says to adjust the throttle to 6mm and they should from 5-6kg/hr but my car runs very poorly at that configuration. At 7mm the car is pulling 7kg/hr on each cylinder. Everything looks great at idle and cruising, WOT wouldn't be impacted and feels the same.

 

So uhhh.. good idea/bad idea to keep it like this? What do the people think?

[savagete2860]

I ended up taking off the RBTBs again to replace all of the o-ring seals. measuring the old o-rings, they were ~50mm ID and 1.8mm CS. I bought a few sizes around this range and ended up using "uxcell Nitrile Rubber O-Rings 52mm OD 48mm ID 2mm Width" that I bought from Amazon.

 

Putting them back on and re-calibrating everything, I started to play with the air bleed valves for the first time. I set the throttle openings to 6mm and slowly cracked open one of the air bleed screws while measuring the air flow rate. This actually INCREASED the the airflow. It makes sense thinking about it and now seeing/hearing it happen, but I was under the impression they would decrease airflow, making my issues worse. Well I adjusted it up to 6kg/hr to stay in the spec recommended by caterham (they say 5-6kg/hr) and the car runs much better than 6mm with the air bleeders fully closed. Still not as nice as it did when the car was adjusted out of spec to 7mm opening and 7kg/hr, but enough that I almost don't notice any of the off idle hesitation issues. Once fully warmed up I am seeing STFT between -15 and -12 which while not ideal is better than pegging -20% and probably wanting to pull more.

 

Still curious if anyone sees any issues with either running the throttle openings slightly more open than spec or at the recommended spec with the bleeders open more. Both set to 6.5-7kg/hr airflow at idle. I think I am calling that issue solved, I don't think there is anything wrong with the car, it just wants more air at idle.

 

The issues I had on my last trackday is now my primary concern. My local track, PittRace, decided to randomly sell in a single day earlier this week. Rumor is for a boat load of money for a datacenter. Really sucks. It is a really awesome and modern track and its going to be a huge loss for the motorsports in my region. So my urgency now comes from having 2 last track days this year - and forever at this track. I really want to be able to drive it for these last times and having the car be undrivable would be a huge disappointment.

 

I have wailed on the 420R on the street and at several autocrosses and it never missed a beat. My first trackday though, the car fell apart. 2nd and 3rd sessions ended with the car cutting power - with not real reason aside from maybe heat. This was at WOT and part throttle, high and low rpm - but mostly with a little bit of load. It also happened on the highway on the drive home aftert he car had some time to cool. The next day the car was totally fine and I have not been able to reproduce since.

 

I am wondering if this is a fuel or ignition issue tied to an electrical gremlin or heat. I have a battery master cut off switch (the CC one) and I have always had a peculiar issue where very seldomly, if the battery switch was off and then flipped back on, the car would crank and never start or even seem like it was trying to. I could crank forever and it would never start. When this happens, flipping the switch off and then back on always fixed the issue and the car would start like normal. I did manage to catch this wile trying to connect easimap - it seemed as if the ECU didn't have power. Not sure if that narrows things down or not for any gurus.

 

Anything else I should check? I feel like I can't do much unless the issue gets worse at least enough to replicate the issue but want to cover any bases that I can. I just don't know where to start.

 

 

[BlakeJ]

I had a misfire at the track that turned out to be a slightly loose battery cable. Forgive me if you’ve already checked this, but it might be worth a quick look. 

[savagete2860]

12 hours ago, BlakeJ said:

I had a misfire at the track that turned out to be a slightly loose battery cable. Forgive me if you’ve already checked this, but it might be worth a quick look. 

 

Thanks for the suggestion @BlakeJ. When you had your issue was the misfire intermittent before fixing? Was it similar to the video I posted?

 

On my car, the cables to the battery are tight and the grounds to the chassis and engine are good. Wires to the alternator and starter seem fine too. I am not ruling out a battery connection issue but I think it would be at or after the master cut off switch. Although I am not sure what is what, the connections on the switch under the dash seem okay. I am not sure how to test and verify that though.

 

[11Budlite]

I would bypass the master cut off switch to rule that out as a potential issue. Not sure what brand Caterham uses but some of them have been known to cause problems.

Edited October 8 by 11Budlite

[BlakeJ]

8 hours ago, savagete2860 said:

 

Thanks for the suggestion @BlakeJ. When you had your issue was the misfire intermittent before fixing? Was it similar to the video I posted?

 

On my car, the cables to the battery are tight and the grounds to the chassis and engine are good. Wires to the alternator and starter seem fine too. I am not ruling out a battery connection issue but I think it would be at or after the master cut off switch. Although I am not sure what is what, the connections on the switch under the dash seem okay. I am not sure how to test and verify that though.

 

Mine (420-spec with roller barrels) also hesitates/stumbles if I give the throttle too big a stab off idle. It always has. But as you note, once under way, it’s not an issue. Really just a nuisance in local stop and go driving,  it a non-issue at the track.

Not sure if that’s the hesitation you’re speaking of. 

[savagete2860]

22 hours ago, 11Budlite said:

I would bypass the master cut off switch to rule that out as a potential issue. Not sure what brand Caterham uses but some of them have been known to cause problems.

I think I would like to have this as an option to try. I might be being daft here, would I simply connect the two battery cables here? There are 4 other connectors on the back of the master switch. 2 go to a weird white rectangular box and two go to what I am guessing is the ignition switch (white and purple wires).

 

 

16 hours ago, BlakeJ said:

Mine (420-spec with roller barrels) also hesitates/stumbles if I give the throttle too big a stab off idle. It always has. But as you note, once under way, it’s not an issue. Really just a nuisance in local stop and go driving,  it a non-issue at the track.

Not sure if that’s the hesitation you’re speaking of. 

What you describe sounds like where I am at now with the air bleed screws adjusted but still with 6mm throttle openings. Its slightly annoying driving around town but not awful. It must just be a caterham thing, any other car I have had does not do this. I really think its just a poor tune that is designed to work in almost any condition without concern for running well, especially at startup/idle. Maybe one day I will get a remote tune on the car.

 

Before the throttle adjustments, my car was sketchy pulling out into a high traffic road, especially from a hill, because you never knew if the car was going to bog down or even stall. Autocross launches were very annoying too. Pressing the throttle to raise rpm to ~3-4k RPM would cause a huge hesitation that would clear up after letting off the throttle and getting right back on it making it feel like a song and dance to get off the line.

[slowdude]

I've been chasing similar ish issues on a separate chassis for the better part of a year and a half.

 

Going back through this:

1. Grounds are good?

2. Lambda is new?

3. Plugs are new?

4. AFR is good?

 

Since the ECU controls timing, and you haven't touched the mechanical timing? You should be fine.

 

I'm re-watching your videos-> your fuel is pulling 20% more to adjust for the lambda.

 

I'm wondering if your fuel pressure is low, and the car is sending fuel pump to push more fuel through.

 

I say that because your plugs were getting fouled despite the lambda saying youre at stoich, and timing we can rule out.

 

I'd rule out the master switch. The car runs normally, it just acts up, if the master switch was toast, you'd have trouble firing up and driving.

 

It leads me to think two things, which I'm also going through.

 

ECU has gone silly, and lost calibration of sensors.

Or, your fuel pump has something going on (fuel trim, and black plugs despite the AFR being "OK").

[savagete2860]

Grounds seem good, I ended up not replacing the lambda sensor, new plugs, unknown about AFRs since I only have the narrow band. When driving, the car feels fine, pulls fine, etc. I actually think the plugs were likely so dark because of how rich it is/was at idle. I tried a new ECU and the issue remained so not an ECU problem. Is there a way to track fuel pressure in easimap? I don't even know if the car has that data available, i think its all just voltage based from the ECU, right? That is one area I feel like I am in the dark.

 

As I mentioned before, the idle, startup, and tip in off idle issue is completely resolved if I introduce more air at idle, over the recommended settings from Caterham. CC say throttle openings at 6mm and pull 5-6kg/hr. If I pull 7kg/hr on each cylinder either via opening the throttle bodies more or from the air bleed screws, I have no issues at all in those areas. The car simply wants more airflow in that area for some reason. To be honest I am just going to run with it and not worry about it unless I ever hear back from CC that its a huge mistake and the car will explode.

 

The bigger issue is how it acted on track and on the way home, where it would have no power for a decent period of time. I don't know what could cause that other than an electrical issue or something that is/was failing. I do kind of wonder if the pump was acting up. Visually all the connections and everything seem fine, and the car runs like a top on the street noodling around, hard on back roads, and even at autocross. What could even cause something like that which would magically fix itself by the next day with 0 change? I have absolutely no clue.

[slowdude]

Ive narrowed my issue down to a faulty coolant temp sensor to the ecu or a capacitor issue in my ecu.

 

I can drive for 20 minutes fine no problem, car will misfire and cut power. Key off and on resets the car and it drives no problem.

 

So its an input issue or an ecu issue. Cant be heatsoak as theres no cool off period, and the car runs fine. 

 

Im the same as you. I dont want a power cut at the worst possible time.

 

Only thing I can think of is its having an issue fueling.

 

Giving it more air via the throttle body indicates a fueling issue -> youre giving it more air to lean out the mixture.

 

If you can, grab a stand alone wideband sensor and gauge and see what happens when the misfire starts.

 

My AFR for example is fine when my misfire happens, so I know its fueling OK and intake is set OK. I check ignition in ecu and its OK too, so it has to be input or control system.

[KnifeySpoony]

12 hours ago, savagete2860 said:

It must just be a caterham thing, any other car I have had does not do this. I really think its just a poor tune that is designed to work in almost any condition without concern for running well, especially at startup/idle. Maybe one day I will get a remote tune on the car.

 

Before the throttle adjustments, my car was sketchy pulling out into a high traffic road, especially from a hill, because you never knew if the car was going to bog down or even stall. Autocross launches were very annoying too. Pressing the throttle to raise rpm to ~3-4k RPM would cause a huge hesitation that would clear up after letting off the throttle and getting right back on it making it feel like a song and dance to get off the line.

 My 420R on roller barrels is nothing like this. It pulls smoothly from idle and has immediate throttle response at any RPMs. Is your car a new/recent build? If so I would press your dealer/CC about it. Are you at altitude? I wonder if that could play a role as well. I would be very frustrated if I were you - if a warranty claim is not an option, I would consider going standalone ECU and having a pro tune it. Will be pricey and possibly inconvenient but at least the car will be right.

[savagete2860]

@slowdude Yeah that does sound different than mine. I can drive my car for short or extended periods of time with no issues. I couldn't even give you a description of why I had misfires at the track, it wasn't that hot and after the car cooled down a decent bit it had the same issues on the way home. Then 0 issues at all in 24 hours. I put on about 3k miles on the car in what has realistically been about 2 months, driving it almost every day it is nice outside, and the only time it had a true "this is totally broken" issue was the track day.

 

@KnifeySpoonyWhat year is your build? I am guessing you are on the 9a4 ecu? Are you running with the green or black injectors? How you describe your car is in line with most owners with RBTBs threads I have read (blatchat mostly). Kind of why I was surprised my new car seemingly ran like a hunk of junk until I introduced more air. You mentioned somewhere your car idles at ~1100rpm. I am wondering if you have an older/different tune. I was told the car should idle between 900-1000 RPM and my car in closed loop will always target that. I have seen 1100 before and that when I added even more airflow which is when I first realized that seemed to fix my startup/idle issue. That was also only in open loop, and close loop pulled timing/fuel to bring it back to 9-1000rpm.

 

My 420 is a new car assembled by RMC. Summer 2024 build (first year they started using the newer 9M4 ECU) that I took delivery off in late June of this year. Josh from RMC has been working with me to help troubleshoot the car's issues. Me being located in PA (not at altitude) and them being in CO makes it hard to really do much aside from send parts back and forth. Aside from the initial "sticky throttles" that are now fixed and mystery trackday misfiring that pretty much ruined the first track day, the issues have been tolerable (and now bandaid-able) to where I would rather keep the car in my possession to enjoy for the back half of the year before winter comes. If the car has the same misfire issues again on my next track day I think my only option at that point will be to ship the car back to CO.

 

 

Related to the idle and start up, below are my throttle adjustments, RPM and STFT evaluated using Easimap.

 

Baseline (hesitation and bogging issues)

Throttle opening size: 6mm
Airflow Rate: 5kg/hr

Air bleed screws open or closed?: closed
Cold idle RPM: 4-700 RPM
Warm idle RPM: 900-1000RPM

STFT correction at warm idle: pegging -20%

 

Adjustment #1 (Started and ran great - maybe a weaker over run feeling and slower rev dropping between gears)

Throttle opening size: 7mm
Airflow Rate: 7-7.5kg/hr

Air bleed screws open or closed?: closed
Cold idle RPM: 1000 RPM
Warm idle RPM: 900-1000RPM

STFT correction at warm idle: +/-5%

 

Adjustment #2 (This is what I am running now.)

Throttle opening size: 6mm
Airflow Rate: 7kg/hr

Air bleed screws open or closed?: open
Cold idle RPM: 800-1000 RPM
Warm idle RPM: 900-1000RPM

STFT correction at warm idle: -12 - -8kg/hr

 

[KnifeySpoony]

My build was 2021. I am on the older ECU, I think it's the 9A4. I am black injectors, even though they sent me the green injectors with my roller barrel kit. My ECU is meant to run on the black injectors even with the roller barrels. My idle is between 1000-1100. 

[savagete2860]

I ended up at the track last week and it went great.

 

 

Rocky Mountain Caterham sent me out a beefier "race" coil subloom that I ran. I don't know if it was the race subloom, cooler temperatures, or something random I tightened since last time, but the car ran great without any issues. Hopefully whatever the cause was it doesn't pop up again. Aside from just going over the car 100 times the changes were:

- Race coil subloom

- Newer Ford plugs

- Swapped to another (aftermarket) TPS

 

I will post an update if it pops up again, but hopefully this thread slowly gets pushed down never to be surfaced again.

 

[KnifeySpoony]

Nice glad it's sorted. BTW what the heck was that red car at 2:05?

[Croc]

Sadly no more Pittrace - last weekend and its done.  

[savagete2860]

19 hours ago, KnifeySpoony said:

Nice glad it's sorted. BTW what the heck was that red car at 2:05?

I think it was a ford fiesta st. Turn 17 was catching a lot of people out that day lol.

 

https://10fsphotography.smugmug.com/Pitt-Race-TNiA-10-15-2025/Novice

 

17 hours ago, Croc said:

Sadly no more Pittrace - last weekend and its done.  

The car community is totally gutted about this in my region. Shameless plug to push back even if the outcome is likely inevitable: https://savepittrace.org/

 

PittRace is an AWESOME place. Alan Wilson designed track, exceptional facilities, hosts everything motorsports related, drifting, karting, autocross, car shows, motorcycle racing etc. The owners were putting about $1m each year into the facility since they bought it I think 2010 or so from the original owners. The sale came out of nowhere and speculation is the owners were offered a boat load of money for a fire sale. The same developers supposedly also purchased an RV park and a golf course for a total of 2k acres. If it does become a data center it will likely be largest one in the world. If it can happen to a track that is doing great financially and owned by a couple who loves motorsports and are already extremely well off, you better believe it can happen to your favorite track.

[KnifeySpoony]

2 hours ago, savagete2860 said:

I think it was a ford fiesta st. Turn 17 was catching a lot of people out that day lol.

 

https://10fsphotography.smugmug.com/Pitt-Race-TNiA-10-15-2025/Novice

 

Yeah I wondered what was it doing out there. It almost looked like it was on a nearby road lol....