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Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 22746
Experience:  BEng hons Mech engineering, in auto industry 22 years
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1990 Lotus Elan M100 - without foot on accelerator, engine

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1990 Lotus Elan M100 - without foot on accelerator, engine will fire up even when cold (just) on 2 cylinders only. Gently depress accelerator with car stationary in neutral and it will go to three cylinders, then at 2.5 to 3K revs the fourth cylinder comes in. THERE IS NO SIGN OF AN ENGINE MANAGEMENT WARNING LIGHT ON THE DASHBOARD. Try to drive the car, and, under load, the engine behaviour is simply too eratic to be safe! During the last 5 years, as they have failed, 2 individual ignition coils on the coil pack have been changed to successfully solve a permanent missfire problem on the relevant cylinder.
Could the present problem be a symptom of the plug leads starting to fail - how does one test them?
If not electrical, only other possible problem could involve the blockage of the drain pipe in the petrol filler well, which resulted in rain/carwash water filling the well. Filler cap tightens till it clicks, but could water have got into the fuel system, and would it create the symptoms experienced?
Thanks in anticipation. David Ashworth.

Hello David

water in the fuel would produce misfires and generally poor running on all cylinders, which sounds like is not whats happening here - however it might be worth trying some 'dry fuel ' additive you can get in Halfords

Check the resistance of the spark plug leads ( 13-15Kohm / metre) and replace if any doubt or high resistance - Also replace the spark plugs if any of them fail this checklist:

You can also try spraying some water on the coil pack with the engine running. It is possible the coil is faulty and causing the misfire. If you see sparks arcing with the water, then replace the coil.

I'd check the condition of the injector plugs / connections as this could be a lean misfire if an injector isn't firing properly - the best way to check this is with a 'noid' light

Then check all the connections to the engine sensors one at a time, remove each one and examine for signs of corrosion or damage and a squirt of contact cleaner ( not WD40) will also help.

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and leans the engine out causing rough running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas around each joint in turn. If the engine rev's up you've found your leak.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Customer: replied 3 years ago.


Sorry I was out yesterday p.m. I haven't had time to fully explore what appears to be a really excellent and hopefully comprehensive list of items to check in order to identify and eliminate the misfire on my Lotus Isuzu engine. Suspect your airleak suggestion may be a good place to start, as some hoses are disconnected to get at the coil pack, and they may have subsequently worked loose.

Thanks for the helpful suggestions, all of which do make sense.


no problem David

just get back to me on this post
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