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TVR Tasmin, Troubleshooting

As mentioned previously, I’m having some running issues and I need to try and work out what it is.

The issue has been getting worse the more I run and drive the car. It previously only happened under load at about 4000rpm, where the engine would fall on its face and cut out like you had turned the key off. If you kept the throttle steady it would recover with a surge as the revs dropped, but as soon as it got up to about 4k again, it would cut out. When stationary, and not under load, it will happily rev to its 6k redline.

It’s a very annoying issue. I had a couple of suspicions of what it could be, but I needed to do some testing to narrow it down.

My first suspicion was that I must be losing spark, as the engine cuts so abruptly and I felt that if it were fuel it would kinda stutter out.

The first thing I noticed was that the replacement coil the previous owner had fitted was a Bosch GT40. This is a 12v coil, and from what I have discovered, widely known to have issues with electronic ignitions. Now, the ignition system on my car is both a ballasted system and electronic. A ballasted system runs a resistor on the power feed to the coil, dropping the voltage, meaning the coil doesn’t see 12v. So I was already running the coil under voltage.

After testing and confirming the resistor was still in place, I removed this coil and refitted the original Ford coil that came with the car.

Changing this resulted to a smoother idle (maybe, but probably a placebo), but the engine still cut out.

The next thing was to check the lead positions, cap and rotor. I note that the cap and rotor have been replaced, and the old ones were with the car. The old cap is stuffed, but the old rotor looked OK, so I swapped that in after a quick clean. No change.

Next was to confirm if I did lose spark or not. The only way I could think of was by thinking out of the box and using my timing light… So I connected it up, and strapped it to the wiper so I could see it when driving. Sure enough, when the engine cut, the light was still flashing away happily. That indicates I have spark (although, not if I have a good strong spark, but some spark is better than none).

So this points me down a different path. I have air, and I have spark. Could it be fuel after all?

I posted up on a few forums asking for help, and got some great tips on where to start looking. One of the easiest to do was to check the sensor plate in the AFM was clean. This plate is hinged, and lifts up with airflow as it gets sucked into the engine, which in turn pushes a plunger up and down to control fuel flow. Well, although the top looked reasonably clean, the underside was filthy (probably thanks to the incorrect air filter that was fitted)

I gave this a thorough clean and made sure it was spotless. This made no change, but its good to know its clean now.

I also, once again, checked the intake tube and boot for splits or cracks, but none found. All vacuum lines (all two or three of them) were checked, all OK.

One thing that makes me suspect its possibly fuel, is the state of the new plugs I fitted. Even after a couple of runs around the block, a couple look almost unused. Maybe lean? From 1 to 6 in order.

This is leaving a couple of possibilities, ones that I cannot test without further equipment. First is an intake manifold leak or the likes. Today my smoke machine arrived, so I could test this.

It’s just a cheapie, but makes a good amount of smoke, and has a pump to actually blow it out under pressure.

After a quick test, I modified it to work how I needed it to. I salvaged a cap from a CRC rust convertor can, drilled a hole in it, glued some hose into it, and glued the cap onto the front of the smoke machine (as it just has a big nozzle on the front normally, for maximum smoke dispersion in da clubz)

This hose then goes into a rubber glove with a finger cut off it. I find using one of these seals the intake pipe nicely with the wrist of the glove, but doesn’t crush the rubber hose.

I don’t know if it’s good or bad, but smoke testing the intake shows there are no intake or vacuum leaks. I tested with the throttle open and closed, to check the intake hose for leaks as well as the plenum/manifold, but nothing, not even a weep.

So what left? Well, two things really. I need to test the fuel pressures of the system, but because the fuel fittings use banjo bolts and hard lines instead of rubber hoses, I can’t use my existing pressure testing kit. I have a new kit on the way, which has all the fittings I need to connect into the KJetronic system. This will tell me if there is an issue with the Warm Up Regulator or the main pressure regulator. The whole system relies on having the correct pressures, so even a few PSI difference can make it all turn to custard.

The other thing I need to do, once I have tested the pressures, is to remove the fuel distributor and injectors. I need to see if the plunger in the fuel distributor is clean and moving freely, or if it’s sticky. I also need to check the flow and spray pattern from the injectors. It’s possible either, or both, of these things are causing an issue.

My main theory at the moment is a sticking fuel plunger, which is causing the engine to starve of fuel when it demands more. The worst thing you can do with a KJet system is to leave old fuel in it and not run the car regularly. This car has been off the road for years, and I don’t know when it was last started and run, or how old the fuel in the system was. KJet runs at such fine tolerances that even a slight gum or varnish on a component can make it upset.

I’m determined to work this issue out and get the KJet working properly as it’s a cool system, and one I haven’t worked with before. I have everything I need to convert to EFI and Speeduino, but I’m trying hard to resist that urge.

Now we wait. Once the pressure test kit arrives, Its game on.

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