Flat spot

[Type40]

Hi peoples. I have a 2000 NM 3.5 Auto and it has a flat spot when first started cold. It has gradually been getting worse since buying the car in Feb this year but it is not at all life threatening, just irritating.

What happens is the engine doesn't respond to the first 1/8 throttle but then pulls away harder than intended. As the car gets to operating temp the problem lessens then disappears completely. I have just had new plugs and leads done (NGK plugs and genuine leads) when the car was put on injected LPG.

If anyone has any ideas where I should be looking the advice would be appreciated.

Thanks!

[psproule]

Does it do it on Petrol as well as LPG? It sounds like the cold start enrichment isn't right.

[Type40]

Does it do it on Petrol as well as LPG? It sounds like the cold start enrichment isn't right.

It only does it on petrol. It starts on petrol and switches over at @55 degrees. So if the coolant temp drops below that it goes back to petrol. And when its warm it drives fine on both fuels

[dolphin]

so what this sounds like is; when the engine or a componant warms up a little all is then fine. unfortunetly this problem could be a number of things from a blocked sensor to a lazy fuel pump etc. when you firstly start up the engine in the morning [ cold ] what rpm,s does the engine sit on for the first 30 secounds ? then what rpms after 5 minutes of idle ?

[Type40]

so what this sounds like is; when the engine or a componant warms up a little all is then fine. unfortunetly this problem could be a number of things from a blocked sensor to a lazy fuel pump etc. when you firstly start up the engine in the morning [ cold ] what rpm,s does the engine sit on for the first 30 secounds ? then what rpms after 5 minutes of idle ?

Bear with me. I will take note tomorrow morning when I go to work. But from memory it is approx 1500 on start up then 800-1000 5mins later.

[Type40]

When I first start the car she goes to 1500 then gradually drops to 1200 within 10 seconds. After 5 mins the idle is 600ish in drive and 700 in park.

[luvthatpaj]

When I first start the car she goes to 1500 then gradually drops to 1200 within 10 seconds. After 5 mins the idle is 600ish in drive and 700 in park.

Yeah, same as mine does too. This is a long shot, but how does the engine sound? Reason I ask is, it could possibly be the exhaust manifold. When cold it leaks, resulting in less compression, then as it warms up the metal expands... Get a metre of garden hose, put it to your ear, and move it around the manifold/gaskets listening for the leak.

[hootnah]

Takr your idle stepper control motor off and give it a good clean out then put it back on, it will take a few restarts of engine for the ecu to relearn and reset parameters. Common problem.
Cheers, Jim.

[Type40]

The engine is as quiet as a mouse Luke. No knocks, rattles, smooth idle. Infact it is quieter than the TL engine!

Hootnah, I take it you are talking about the idle stability and not the flat spot?

[John McFarlane]

Luvthatpaj, how does a cracked manifold lower compression?

[hootnah]

The engine is as quiet as a mouse Luke. No knocks, rattles, smooth idle. Infact it is quieter than the TL engine!

Hootnah, I take it you are talking about the idle stability and not the flat spot?

yep.
Cheers, Jim.

[luvthatpaj]

Luvthatpaj, how does a cracked manifold lower compression?

My bad, had a few too many. I meant to refer to back pressure.

[Type40]

I think it may be nutted out. I had the engine temp sensor replaced and the flatspot is 99% gone. All we need is a really cold morning to suss out just how "fixed" it is!

[Type40]

I think it may be nutted out. I had the engine temp sensor replaced and the flatspot is 99% gone. All we need is a really cold morning to suss out just how "fixed" it is!

Not fixed. This thing is doing my head in! Also, it is now worse. On LPG it is 100% fine but it has now started to do it slightly when warm on petrol. And at a cold start it nearly stalls. If anyone can help it would be greatly appreciated!

[Type40]

Just letting people know that the issue is now sorted.

It appears that there was a 3 prong sparkplug that Mitsubishi used to remedy the "feel" of a flatspot in the NM 3.5's. I have now fitted these plugs and all is well.

So when replacing the plugs on your NM don't use the recommended ones in the manual, use the ones with the 3 electrodes as made by NGK.

Part number is MN119942 or BKR6ETUB