620 S Build

[Dave W]

I would check your build manual, for the exhaust system. I do not think you want the O2 sensor facing down, because it can collect water.  

[MV8]

I agree Dave, but they do that now to hide it imho. Not good for longevity of the sensor as water, oil, and fuel can collect in it on shutdown.

 

Caryenta, closer pics of the induction system please to show your throttle control  (ITBs or a single throttle body and if you have an IAC valve).

[CarYenta]

On 11/8/2025 at 4:42 AM, MV8 said:

I agree Dave, but they do that now to hide it imho. Not good for longevity of the sensor as water, oil, and fuel can collect in it on shutdown.

 

Caryenta, closer pics of the induction system please to show your throttle control  (ITBs or a single throttle body and if you have an IAC valve).

Single throttle body:

[CarYenta]

On 11/8/2025 at 4:25 AM, Dave W said:

I would check your build manual, for the exhaust system. I do not think you want the O2 sensor facing down, because it can collect water.  

There is only one orientation that would fit with the pipe bend and two spring attachment points for the 4 into 1 pipe. It appears the OEM literature shows it pointing down as well. Hopefully that won't be an issue!

 

Edited November 10, 2025 by CarYenta

[CarYenta]

Well I've bled the brakes 4 times, there still seems to be air in the system. I managed to melt a hole into my power bleeder hose which somehow ended up resting upon the exhaust also 😆

 

Also noticed my fuel gauge reads empty even though I just filled it. Should I check the sender on the tank?

 

For the red light between the tach and speedo, I charged the battery with a CTEK MXS 5.0 for the agm battery and the light was no longer illuminated upon turning the key, so that was a correct diagnosis by MV8. 

Edited November 10, 2025 by CarYenta

[MV8]

Looks to be Alpha-N ecu if there is no MAP sensor. No IAC shown so idle is set by screw and no compensation for hot or cold running but also cannot cause rpm oscillation you've described. You didn't say what you set the warm idle speed to, but I expect it is too high, putting it in a different position on the ECU tuning map where the timing is changing. Do not loosen and play with the TPS setting without having a spec and a DVM to check it. Assuming for now that the TPS is adjusted correctly for the TB butterfly idle position, try to reduce the warm idle setting to around 800 rpm. This means you will need to keep the throttle open slightly during cold start long enough to stay running.

[CarYenta]

4 hours ago, MV8 said:

Looks to be Alpha-N ecu if there is no MAP sensor. No IAC shown so idle is set by screw and no compensation for hot or cold running but also cannot cause rpm oscillation you've described. You didn't say what you set the warm idle speed to, but I expect it is too high, putting it in a different position on the ECU tuning map where the timing is changing. Do not loosen and play with the TPS setting without having a spec and a DVM to check it. Assuming for now that the TPS is adjusted correctly for the TB butterfly idle position, try to reduce the warm idle setting to around 800 rpm. This means you will need to keep the throttle open slightly during cold start long enough to stay running.

 

Thanks! I have idle set to right about 1000 now and it seems pretty steady, but has some significant delay in response. If I move it up to 1200 or so, it has much faster response but then it starts oscillating between 1200-2000 rhythmically when warm. Would the PGM ecu help fix this? I think Beachman said he's a vendor of theirs.

 

To cold start the car, I seem to require excessive cranking. 5 sets of 10 seconds of cranking, then wait 5-10 minutes, then it starts right up. Would having the throttle very slightly depressed help this, or hurt this? I'm worried about flooding but I also don't exactly understand what's happening when starting a car with the throttle depressed slightly.

[MV8]

Don't expect this to have the same good drive ability as a mass produced car with modern efi. This is a race car with a very basic efi where the clutch is not intended to be engaged until higher rpm. In some ways, AlphaN is more limited than a carburetor to address drive ability issues due to a lack of common efi sensors that all other systems incorporate.

Waiting assumes flooding occurred but it may be lack of fuel. Initially, I would turn the key to RUN and wait three seconds for the fuel system to pressurize, followed by engaging the starter, foot off the throttle until it starts or five seconds. If it doesn't start, and if I don't smell (unburned/flooding) fuel, I'd immediately repeat the process. Holding the throttle open while cranking might help with excess fuel/flooding but not for a lack of fuel issue.  

[CarYenta]

38 minutes ago, MV8 said:

Don't expect this to have the same good drive ability as a mass produced car with modern efi. This is a race car with a very basic efi where the clutch is not intended to be engaged until higher rpm. In some ways, AlphaN is more limited than a carburetor to address drive ability issues due to a lack of common efi sensors that all other systems incorporate.

 

Waiting assumes flooding occurred but it may be lack of fuel. Initially, I would turn the key to RUN and wait three seconds for the fuel system to pressurize, followed by engaging the starter, foot off the throttle until it starts or five seconds. If it doesn't start, and if I don't smell (unburned/flooding) fuel, I'd immediately repeat the process. Holding the throttle open while cranking might help with excess fuel/flooding but not for a lack of fuel issue.  

 

Ok - I had only 2 gallons in the tank the first two times cold starting from a jerry can, but have just filled up with 9.5 additional at the gas station. Maybe the additional fuel will help. Does holding the throttle open give additional fuel, or only additional air? Will it be more likely to flood if held open?

[MV8]

The level in the tank has no affect on the cranking mixture.

I don't know. I think it would be more likely to flood. It depends on the programming. A normal vehicle typically cuts fuel at WOT during cranking to clear a flooded condition.

 

You can always pull a plug to confirm.

Perhaps a video with sound of your starting attempts from sitting/cold?

Does it crank easily when warm?

[CarYenta]

56 minutes ago, MV8 said:

The level in the tank has no affect on the cranking mixture.

I don't know. I think it would be more likely to flood. It depends on the programming. A normal vehicle typically cuts fuel at WOT during cranking to clear a flooded condition.

 

You can always pull a plug to confirm.

Perhaps a video with sound of your starting attempts from sitting/cold?

Does it crank easily when warm?

I saw some first-start recommendations to have 5 gallons in the tank, maybe there is some initial priming pressure that helps with which no longer applies to me.

 

When cold, it cranks rapidly without firing, sounds perfectly normal cranking, just zero firing at all. The two times I've started from cold successfully, I've waited several minutes after the initial cranking disappointment, and after that waiting period, it fires immediately and crawls up to a grumpy idle speed about 1000 rpm.

When warm, the engine fires immediately without delay, but again nearly bogs until garnering strength to hold at about 1000 rpm.

[MV8]

If you have an MBE ecu, get the software and cable to see what is going on with a lap top as a diagnostic tool:

 

[MV8]

I still don't know what it should be, but I found that some MBE Jenvey efi kits are set up for an idle speed of 1200 to 1375 rpm while keeping the TPS voltage at between 0.30 and 0.37vdc and 44.1 psi fuel pressure. A CAT dealer should have access to the correct, basic servicing information. For now, you could raise the idle a bit but keep it under the lowest rpm at which the warm oscillation occurs. Higher idle speeds can make engagement of first or reverse more difficult.

[CarYenta]

15 hours ago, MV8 said:

If you have an MBE ecu, get the software and cable to see what is going on with a lap top as a diagnostic tool:

 

My ECU has written on it "30L279B", and looking at the Caterham parts page, the URL says 9M4 https://caterhamparts.co.uk/ecu/8146-ecu-9m4-duratec.html, so I'd assume it's an MBE branded ECU. Is there a cheaper ebay version of this cable?

 

[MV8]

Unfortunately, the cable contains electronics that make it compatible only with MBE software, which also make it more expensive.

[Croc]

@CarYenta I'm late to this thread but...I see a brand new 620R.  Who is your dealer?  Beachman Racing?  Bruce has a engine dongle cable - I saw it on his bench two weekends ago when I was in the workshop with Bruce assessing the state of my green boat anchor.  If you ask nicely you may be able to borrow it. 

 

However, I am wondering if your kit suffers from the traditional issues with as delivered 620 kits - overcooling, map issues, need to replace TPS, etc.?  I thought Caterham fixed it in the recent kits?

https://www.pgmsussex.com/upgrade-kits

 

Bruce would be the person to have a quick call with to confirm if you have the 620 known faults.  He is very familiar with PGM and Andy Jupp.  Andy is a top guy - I would recommend him.  

 

If you bought through Rocky Mountain then Josh also knows the drill with the 620.  Even Christopher John Caterham in FL (or is it GA?) knows about it as I just came across one of that dealer's cars with the fixes.  

[CarYenta]

8 minutes ago, Croc said:

@CarYenta I'm late to this thread but...I see a brand new 620R.  Who is your dealer?  Beachman Racing?  Bruce has a engine dongle cable - I saw it on his bench two weekends ago when I was in the workshop with Bruce assessing the state of my green boat anchor.  If you ask nicely you may be able to borrow it. 

 

However, I am wondering if your kit suffers from the traditional issues with as delivered 620 kits - overcooling, map issues, need to replace TPS, etc.?  I thought Caterham fixed it in the recent kits?

https://www.pgmsussex.com/upgrade-kits

 

Bruce would be the person to have a quick call with to confirm if you have the 620 known faults.  He is very familiar with PGM and Andy Jupp.  Andy is a top guy - I would recommend him.  

 

If you bought through Rocky Mountain then Josh also knows the drill with the 620.  Even Christopher John Caterham in FL (or is it GA?) knows about it as I just came across one of that dealer's cars with the fixes.  

Hi Croc,

Yes, I bought new via Beachman Racing. I talked to Bruce today on the phone and we're looking into getting the PGM ecu kit. He made a good point to try and get it working normally before adding new variables into the mix. Since it has started from cold, I'm hoping things will fix the self a little and it will start from cold faster. Just last night it started quite rapidly compared to earlier, so I think the 7 miles of driving I've put on it (or the extra 10 gallons of fuel) have done something beneficial.

 

I'll probably end up getting a communication cable anyhow to be able to look at data now and then.

 

For overcooling, I thought that was fixed via the new black plastic thermostat with bypass line compared to the old metal no-bypass thermostat?

[Croc]

Just now, CarYenta said:

I talked to Bruce today on the phone and we're looking into getting the PGM ecu kit. He made a good point to try and get it working normally before adding new variables into the mix. 

 

Yes that would be the best approach.

 

1 minute ago, CarYenta said:

Just last night it started quite rapidly compared to earlier, so I think the 7 miles of driving I've put on it (or the extra 10 gallons of fuel) have done something beneficial.

 

Good stuff.  

 

2 minutes ago, CarYenta said:

I'll probably end up getting a communication cable anyhow to be able to look at data now and then.

 

Best spot to buy is SBD Motorsport.  They are sole suppliers for MBE ECUs.  If you buy from anywhere else you then they have bought from SBD and are reselling.

 

2 minutes ago, CarYenta said:

For overcooling, I thought that was fixed via the new black plastic thermostat with bypass line compared to the old metal no-bypass thermostat?

 

Thats what I thought but I suddenly wondered if you got a pre-factory-fix kit.  

 

 

 

[CarYenta]

Car didn't start again today. Tried starting it a dozen times over 12 minutes with zero firing. I then waited 10 minutes exactly with the key in but not on run. Turning the key to run, the light bulb between the tach and speedo illuminated. This has illuminated every time the car has successfully started from cold. And sure enough, the car started!

 

I also noticed that about 250 rpm registered prior to firing noises were heard. Should there always be an RPM indication when cranking even if the car isn't firing? For the first 12 minutes of trying to start, the tachometer didn't move at all (checked on video).

 

 

[MV8]

This is all making sense now. With the key turned to RUN, you should have multiple indications of life such as working turn signals, the digital odometer showing in the speedo, warning lights in the tachometer, and the big red light between the two instruments. It sounds like one or a combination of factors related to power distribution or grounding; something is not fully plugged in, the ends of the battery cables are loose, or the battery is not fully charged.

The battery should be fully charged to the green lights (step 7 or 8), not just to the "ready to use" light (step 4). Small batteries reduce weight at the expense of allowed attempts to start the engine, so you may just need to charge. A few minutes running (alternator charging) is not long enough to replace a single cranking attempt, much less multiple. The battery is likely undersized to save weight (fewer amp hours) and less tolerant of failed start attempts. 

Battery terminals have a dull grey oxidation when new. This is removed with a wire brush before the cables are attached. I don't know what type of terminal you have but there are brushes for round post types at your local auto parts store.  

I see you have a short neck on the block where the thermostat goes instead of the long metal tube that did not accept a thermostat so you likely have the improved kit.

 

Also, never leave the key in RUN and engine off with out attempted starting within a few seconds. This can overheat the ignition coils and cause them to fail early.

Edited November 12, 2025 by MV8