Hesitating..

[Tobs]

When I’m accelerating my car (1995 Chim 400) seems to hesitate until it reaches about 3200rpm - it then takes off.... Could this be something to do with the lambda sensors? As I know the car runs closed loop up until about 3100rpm? Also having a few probs between 1000-2000rpm when going around town, where it seems to occasional 'miss' and be hesitant when I press the throttle. I have recently checked the lambda’s though and they seemed to be giving out the correct voltages? Could the ignition amp create similar symptoms? (I have recently replaced the leads, dizzy cap, rotor arm and coil).

Cheers,

Toby.

[tahiti450]

Problem with this is that it could be almost anything. Assuming that all the things you've checked or replaced are ok then - My list of suspicions - in no particular order - would include:
Throttle potentiometer (esp given the hesitation on throttle opening),
Coil,
Ign amp,
Main ECU connector (loose / corroded contacts),
Airflow meter,
system voltage,
poor or intermittent earthing problem.
The odd thing is that you say its mainly lower rpm, I'd expect ignition system components to give more problems at higher engine speeds, so maybe less likely in this department.
Worth checking the vacuum advance, for sealing operation and security (as the capsule often works loose on the distributor).
Air leaks on the plenum chamber or anywhere upstream of the AFM (in particular check the flexible hose between the AFM and the throttle body).
What's it idle performance like, both through warm up and at running temp?
More questions than answers I'm afraid!

[andy-w]

I had a similar problem with mine - 94 4ltr - i was struggling to keep it going below 2k revs but after that it would fly along...i changed the rotor arm for a halfords one and things improved...along with changing the coil, leads, ignition amplifier and plugs and it ran a lot better and then things went t**ts up again and it was the c**p halfords rotor arm that was causing me problems.

Tahiti is right, it could be anything...have you got a good spark from the coil, is there any loose hoses going to the airflow meter / plenum...one thing i have found is these engines are very picky!!!

[mowog]

I've just fitted new Lucas OE Dizzy cap, rotor arm & HT leads to my 4.0. (bit of a Christmas present to it!). Difference was amazing - smoother running, pulls in low revs in all gears, no hesitation or lumpy misfire I occasionally had. Just goes to show it is worth keeping ingnition system in tip top condition.

Decided to go the whole hog and buy a new coil today. Went to Halfrauds - usual question 'what car is it for?' told them the lucas part number and they had one in stock. Best bit was when it went through the till it came up as £6.36 - Result!!

D

[Tobs]

I’ve checked the throttle pot on an oscilloscope - seem nice and smooth no spikes or anything. So I think this is OK. As I said it has had a new coil, dizzy cap, rotor arm, and ignition leads - these generally made things a bit better - but no vast improvement. Fuel pressure also seems OK. I'm now perhaps suspecting the ignition amplifier - I will have to see if I can find another one to test and see if there is any improvement. (although wouldn't this generally give more problems at higher revs?)

Toby.

[vapy]

mine went like that not long after i had it so asked mate who i bought it off and it was leaves and crap sucked up the air intake behined the front grill so every couple off weeks i clean it out dunno weather it is on yours but i hope this helps you

[Tobs]

Thanks - I know that there is a round foam filter behind the grill - I'll have a look at it.

[TVR+Bubblecar]

Have you checked for air leaks ?

cheers
Chris

[prbw]

Interesting symptoms, Tobs - my 96 400 is exactly the same! It idles fine, is no problem around town and in general running it drives smoothly, but on full-throttle acceleration it seems a bit hesitant, particularly between 2500 and 3200, when it takes off. Above 3200 it runs cleanly right up to the cut-out. It feels just like a peaky engine coming on the cam - but all the power read-outs I've seen suggest a fairly linear delivery, which mine certainly isn't. I've checked the Lambdas, plug leads are fairly new, as are the plugs, throttle potentiometer checks out correctly, vacuum advance also working properly and no air leaks to be found (yet!). Anyone else had this oddity and cured it?

[mowog]

I had similar symptoms until fairly recently. Decided to give the whole ignition/HT a makeover. New Lucas rotor arm / dizzy cap / leads / coil. This seemed to makes things smoother but I still had a slight hesitation / misfire. The biggest difference came when I then changed the plugs and plug extenders ( www.actproducts.co.uk ) . Wow! - pulls smoothy in all gears from 1200rpm, tickover more stable and a dream to drive again.

D

>> Edited by mowog on Thursday 22 December 18:10

[Tobs]

I've also changed the plug extenders (as well as everything except the ignition amplifier on the ignition side of things)- but I haven’t got round to changing the plugs though yet - think this maybe the next step ( they looked quite new though when i took them out to check them recently though). They were champion ones though - NGK are the recommended ones aren’t they - or doesn’t it make much difference?

Toby.

[mowog]

I was told only to use the NGK B7ECS as they are a competition plug. The B7ECS plugs that came out of mine did not look too bad but you cannot see the condition beneath the electrode as there is a deep 'skirt' on the plug. There were definately the main cause of my poor running - which started after I flooded the engine a few weeks back. I would try the B7ECS plugs next....

Good luck!
D