Hi all,

My Zed is suffering from a misfire/hesitation which appears when the engine is under load and between about 3500 and 4500 revs.

Sitting in neutral, you can rev smoothly and freely without any problems but when accelerating hard, the misfire appears. On several occasions I've been able to "drive through" the hesitation but it's not often that that happens.

 

Zed is a 1993 TT Auto, Zedworld chipped ECU, decats with Mongoose rear, Apexi air induction, HKS ALC, S2 PTU.

 

The car is in good condition and passed it's MOT recently with no advisories, as long as I don't give it too much "welly" it drives as normal.

 

Any pointers on what to look for would be much appreciated.

When was the last time the plugs were changed and gapped? It could be blowing out the spark under load but I'm sure someone with more knowledge will be along shortly

The spark plugs are about four years and four thousand miles old, so I wasn't looking at them as being the cause of the problem, but it is a Zed after all and that means that you can't really rule anything out :laugh:

I have just recently been suffering with a similar situation, I have replaced the plugs, cleaned and checked coil pack and injector connectors etc. with no success. I was trying to pin it to one cylinder, but couldn't. I also changed the PTU and checked the MAF. No joy.

Anyway, after reading various other threads, one suggestion was a/the o2 sensor/s could be faulting causing a poor mixture and misfire type symptoms. It was suggested to try disconnecting them as the car will run fine without and operate on an ECU average value.

I have disconnected them, problem disappeared immediately and has not yet returned. I have done several good trips in the car since and all is well! It is probably only one sensor, can't be bothered to find out which, I'll just change them both in the near future.

A very easy thing to try! You should probably notice straight away.

http://www.300zx.co.uk/forums/showthread.php?177609-Misfire-low-revs-light-accelerator-injector

O2 sensors will only cause problem on part load, should be fine at full throttle...

Do you know if the o2 sensors are ignored at any rev range under full load or is it only at high revs & full load when the system goes open loop???

He hasn't stated full throttle, just thought it worth a try?

The misfire appears mainly on kickdown (full throttle) right enough, under normal acceleration the car drives as normal.

Suppose that I'll have to drive sensibly until I can figure out the cause of the misfire :rolleyes:

 

I'll try disconnecting the O2 sensors anyway, It's an easy fix if it works.

Sadly then its probably not the o2 sensors afterall. Have you swapped out the PTU to check that?

i had exactly the same, I ended up putting in a new loom , series 2 ptu, new plugs , new maf, that cured it , so would suggest you start with the cheapest of these and work upwards, I also checked the TPS voltage, each coil pack voltage, each injector voltage, water temp sensor. think that's about it.

 

good luck

Hi, if you have cleared sprk related possibilities then I would look at the vac line to your fuel pressure regulator, if the vac line is split/off you will not get higher pressure in the higher/in boost range which will give you the hesitation, meaning you will be running lean.

Could be a failing fuel pump control unit, or pump.

Good luck.

Well, I disconnected both O2 sensors yesterday afternoon before I had a 30 mile round trip to do and there was no appreciable improvement in the misfire/hesitation problem :sad:

 

However today, once I had reconnected them........................the misfire etc. has gone :dance:

 

The engine is revving smoothly with no hiccups at all, it stalled once and idle is not perfect but hey.....no misfire.

 

Hopefully it will stay that way but who knows?

 

Thanks for all the advice guys - I grinning all the way through kickdown :laugh: