Talk Morgan Forums Morgan Sports Cars Morgan Three Wheeler & Super 3 Intermittent mis-fire and questions on Protune

Just back from a 1000mile tour of Ireland which was troubled by an intermittent misfire


Brand new NKG Iridium plugs and fresh new Magnecor leads did not fix it.

Symptom is a misfire typically at 2500-3000 rpm starting after a period of some minutes steady running on a light load (straight/level highway - i.e. lowish engine load)

We find that the problem seems to be cleared for a while by giving the accelerator a decent boot and/or "driving it like you stole it" (not really a sustainable solution compatible with retention of licence)

Am suspicious that the TPS (throttle position sensor) may be the cause with a worn bit of track on the potentimeter around mid throttle giving dud info to the ECU...booting the throttle might be temporarily clearing debris on the track...

On return home, have fired up protune but not found any immediately recent historic Diagnostic Trouble Codes other than one reported ~1000 miles ago saying P0122 Throttle Position Low...this was around the beginning of our trip so may mean something and could link to the TPS. The 2/3 other historic codes were much older.

Another oddity was apparent....the info/data record says the engine has done just over 14k miles .... and says it is the original ECU build with no change/update. This bears no relation to the toal mileage for the car nor does it match the mileage since the chassis was changed in 2015 (although closer to this which is around 12500 miles ago). We have not had an ECU change...it is I suppose possible that when Morgan rebuilt the car to replace the chassis, they may have started with a different ECU/wiring loom already installed in the new chassis???

Has anyone any experience on the accuracy of the mileage count in protune??


Looking at the ECM gauges with the angine running, nothing obviously wrong is showing up...and the O2 sensors (another possibility for causing misfires) seem to be reporting fine. However, in connection with my suspicions on the TPS, I was a bit surprised to see the throttle setting at 13% and 0.43V at idle....however, I have no reference to know if this is right or not....

Grateful for any thoughts/suggestions before I start a round of changing sensors and yet another set of plugs/leads.....thanks!

I assume that you have the new Walbro fuel pump installed so no point in that being explored.

The throttle position comes into play on trying to start the engine is my understanding but nothing else except maybe idle.

I bet the accuracy of the mileage is based on the MMC settings for tire diameter. The mileage and speedometer are a joke and is the reason I use a GPS system to log mileage and speed. The MMC reads fast on speed which in my way of thinking means miles traveled would be higher.

Are you topping off the fuel tank when you fill it or are you just filling to one click at the pump?

Sounds similar to a problem I had on my car. Very frustrating - sometimes the car would run for 1hr+ with no problem. Other times it would misfire almost immediately. Misfire always on steady throttle opening - give it the beans and it would pull fine. Turned out to be a faulty TPS. A work-around was to 'exercise' the throttle by depressing the pedal fully 10 times before starting the car. This temporarily cleans up the potentiometer within the TPS (much like a scratchy volume knob on a transistor radio) and effectively made the problem go away.

There are two types of TPS fitted to our cars. Most have the type with slots which allow the sensor to be adjusted before being bolted into place. Unfortunately, you will probably find that these are no longer available from Morgan dealers (although I haven't checked recently). They are available here:

Also, there is a Cycle Pro equivalent that Harley dealers might stock, although it's significantly more expensive than the AliExpress one.
The later (non-adjustable) TPS should be available from Morgan dealers. Once again - not cheap!

I have had exactly the same issue, my car is July 2021 and now covered 20,000 miles, it started random misfiring at about 5000 miles and likewise if you booted it it would clear. It seemed entirely random sometimes every day other times not for 2-3000 miles. The EML also started to come on but not at exactly the same time, fault was PO122 TPS. When checked with Protune it was always a historic code and never current, so I think it was just an instantaneous blip. As it was still under warranty the dealer said the TPS needed adjusting, 8000 miles on it was still doing it and last week they changed the TPS, under warranty, despite it testing as OK. I wonder if it was just a bad plug connection, only 400 miles since and is OK but too early to really tell.

Thanks Andy & Rog for your comments/thoughts...will try a new TPS as you seem to have had (superficially at least) similar problems....

Lightspeed - yes we have the Walbro fuel pump in for a while now and we are careful not to overfill (never encountered that problem). Agree ref the mileometer readings and the discepancy could well explain the difference between the engine number and the mileage since the chassis was changed....I remain suspicious that that process got us a different/new ECU as the factory were doing a lot of cassis rebuilds around then and starting with a wiring loom aready in th new chassis may have been the expedient solution.

(Our car is something of a "BITSA" anyway as it was one of the first early 2012 customer ones (rather than dealer ones) and we found that it was fitted with a set of the prototype Suplex damper spring units supplied by DarkArts to SSL who did the original suspension work....one of the damper shafts broke and we were lucky that DarkArts had another of the matching prototypes lying in their store room....)

I have had similar problems. I cleared the code with Protune, a squirt of contact cleaner around the TPS plug and a whoosh of compressed air and it seems to be OK for a thousand miles or so. I got the P0112 Throttle Position Low to repeat with the Protune instrument screen once and the TPS did dip below 10% at idle for a second or so, that triggers the EML and the idle stuttered at the same time, then resumed a steady idle with the indication at around 12-13%. I guess it will be a TPS change at some point, with the anti tamper screws being replaced.

i put the larger air filter from S$S on. went away when I went back to the standard filter