Hi

 

I know there's loads of threads about the stumble/hesitation issue that seems to be a common problem in the 300ZX, and I think I've read them all, and tried most of the suggestions within them, but I'd like some advice please.

 

I have a 1992 UK 300ZX twin turbo, with a +70BHP EEPROM upgrade and Apexi air filter.

 

The previous owner had the car for 9 years and had the EEPROM upgrade done when he first bought it. He thinks the stumble first started about 4 years ago but as he 'drove her like a granddad when she was cold' never really bothered about it.

 

My particular stumble/hesitation ONLY occurs in that first 10-15 minutes. Once the car is fully warmed up, it doesn't happen at all.

 

It seems to happen mainly when I'm doing 30mph or so in 2nd, or 40-50mph in 3rd, and the revs are between 2500 and 3500. It feels as if someone has taken the accelerator away for second, then it comes back. It may do it once or twice in a few seconds then it'll be normal.

 

Seems to do it mainly when I'm at that constant speed (30mph zone) but occasionally will do it when accelerating.

 

Even in those first 10 mins or so when it's still cold, it doesn't seem to happen at higher speeds/high revs.

 

Every so often it's more severe, and the 'drop off' will last longer than a second and feel likes it's going to stall. It did this really badly tonight. The car hadn't been used since this morning (2hr run, little bit of stumble in first 10 mins then absolutely perfect). I took it to the shops 9 hours later, 10 min journey. Stumbled maybe four or five times, not too deep, but annoying nonetheless. Shopped for 10 mins, came back, drove for another 5mins to another shop, couple of minor hesitations. Shopped for 20, maybe 30 mins. Got back into the car and headed for home, about 2 mins in had major, deep, drop offs, felt like it was kangarooing - would get 2 second without power, 2 seconds normal, repeated, probably about 10 times. All the time my foot was on the pedal consistently. Came to a roundabout and stopped for a sec; pulled away and was fine again.

 

Outdoor temp today has been 12 degrees, so not that cold really.

 

I've been following this document for the last couple of months and trying different things out:

 

http://www.zhome.com/ZCMnL/PICS/hesitation/hesitation.html

 

So far I've done:

 

Unplugged everything - injectors, all connections, cleaned all contacts up

Checked PCV hose/values (clamped as best I could, not fully closed, but there was no fluctuation at all)

Air flow meter - voltage fine

Air filter looks clean enough

Cleaned throttle intake valve

Voltage checked TPS - adjusted to 0.47V

Cold start temp sensor voltage check (readings here http://www.300zx.co.uk/forums/showthread.php?165253-Coolant-Temperature-Sensor&highlight= which I think are fine)

Checked spark plugs - have the correct PFR6B-11C, they look relative new/clean

Replaced Series 1 PTU with a Series 2 PTU

 

There are a few more bits and bobs that I could chec:

OHM check injectors

Replace fuel filter

Fuel pressure tests

 

BUT, reading through that doc above, and other posts, they don't sound relevant to my particular problem - i.e. the fact that the stumble is only when cold (in those first 10-15 minutes).

 

Any more ideas anyone?

 

Reluctant to spend any more time/effort/money on her!!!

 

I AM off to Zedworld in Birmingham at some point in the next month or two to replace an air compressor, I could ask them to take a look, but fear it may take them a long time (and cost me a lot!) to diagnose.

 

I thought the EEPROM upgrade could be to blame but it was installed 9 years ago and it's only been happening for the last 4 years...

 

Thanks

 

Ross

Hi there matey,

I had a very similar problem on mine and it turned out to be a vacum leak through the clutch vacum booster under the dash, was very hard to find untill the leak went much worse, is yours a manual?

i know its clutching at straws but you could do much worse than taking off the pipe to the clutch booster and sucking to see if your getting a vacum and while your down there do the brake booster too, worth a try matey and it will eliminate them from your problems if its not that....good luck :)

Another idea to try - I had a similar sounding issue, that occurred only in the first 5 minutes with a cold engine. I had hesitation and stumbling/kangarooing at part throttle, particularly noticeable just coming on or off the throttle, but also with constant small throttle openings. As soon as the car was warm there was no issue. I found a dead spot on the TPS and got another one from MarkDerby (thanks!). The replacement TPS seems to have fixed the issue.

 

With my original TPS, I could adjust it to 0.45v with closed throttle, but as I slowly opened the throttle the voltage would then jump around a lot as it went through the bad spot, then would be smooth from 1/4 to full throttle. I also couldn't adjust it so that the hard idle switch was closed when the voltage was 0.45v (but I believe the ECU doesn't use the hard idle switch if the TPS voltage is working). So I think the ECU was seeing a noisy/random throttle position at small throttle openings. There were no codes set but you could see the non-smooth pedal response in conzult.

 

I AM off to Zedworld in Birmingham at some point in the next month or two to replace an air compressor, I could ask them to take a look, but fear it may take them a long time (and cost me a lot!) to diagnose.

 

Ross

 

 

Cannot see a better time to sort it, as any of our customers know we do not charge hourly rate as that is the route for main dealers and lesser garages in my view that are not confident enough of their knowledge and experience to do fault finding for a fixed fee.

 

Jeff TT

Edited November 12, 201213 yr by JeffTT

Let me know which one you bought and I'll see if I've got the drivers

 

It's this one and as HL-340 stamped on it, I have googled it and plenty of others with same issues, I did try last night downloading but my Kaspersky was not happy with me as I was getting 'threats' :shifty:

 

This ad does state 'Chipset solution: PL2303', but when I download the Prolific driver it recognizes the cable chip inside the convertor as fake :sad:.

 

http://www.ebay.co.uk/itm/160853614556?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

 

Are you then saying something as cheap as this could well work?

 

http://www.ebay.co.uk/itm/USB-2-0-to-RS232-Serial-DB9-9-Pin-Male-Adapter-Cable-PC-/121001775806?pt=UK_Computing_Parallel_Serial_PS_2&hash=item1c2c4492be

Thanks Jeff - think it was your name the previous owner (Glenn) mentioned too - think you've got an air compressor waiting for me too - will give a call tomorrow!

I was soooo close!!

 

It was the throttle position sensor after all; I'd corrected the idle speed voltage and checked the top end and seemed okay.

 

Took it down to Zedworld today and Jeff was able to see straight away on the diagnostics that the issue was with the TPS and checked out the voltages again; it seems there was an intermittent issue (damn, I should have checked more than once!!) and on a few occasions the voltage would shoot through the roof as the revs increased!

 

New TPS - issue fixed.

 

Thanks Jeff, you're a star!!! :bow: