mx5cartalk.com

Hi all pretty new t the forum and MX5s in general, more of a Honda head but enjoying ownership so far.

I recently bought a very clean 1990 NA6 with 205k on the clock. I have it now for about 6 weeks and gradually cold running has gotten worse.

It starts fine and idles with no problems, but the throttle response seems really bad. As in if I totally stamped the pedal to the floor it would almost stall the engine until it picked up about a second later. It's pretty annoying because I have to keep the revs up slightly when at junctions or traffic lights.

I flashed the diagnostic plug and got 3 codes, namely 3, 15 and 17.
Cam angle sensor seems to work fine and I cannot imaging the O2 sensor would have such an effect on throttle response although it is on my to do list with it's next service which is soon.

Anyone have any experience with this before or have similar issues?

Car is totally stock apart from an exhaust system and a K&N panel filter.

Thanks all...

Would normally say it's one of the two coolant temp sensors at the rear of the head. One CTS is for the dash temp gauge, while the other CTS is for the ECU. It's the latter that frequently becomes unserviceable on the NA6, however it usually also effects cold start idle, i.e. from cold start to warm up it can want to stall. Also, on the NA6 there's a third CTS which is located on top of the thermostat housing at the front of the head for the fan. There was someone else with a similar issue that ended up being caused by something different - I'll have a quick look for it, but here's a pic of the two CTS at the read of the head (small black one with arrow is the gauge sensor):

If you can be bothered trawling through my walls-o-text in the idle dip thread (below this one at the time of posting), although it's dealing with an NB, I was experiencing issues with O2 sensor which most definitely did affect the throttle response. Some info may be relevant to your issue.

Having said that, don't listen to me over 93_clubman - he was the one who helped me understand what was going on

So when it is warm it runs perfectly?
If it consistently run poorly, I'd say......SPARK PLUG LEADS!

i notice sometimes my car will idle strange when the fuel mixtures randomly start to lean out at idle, AFRs from 15.5-16.. i think its the 22yr old 02 sensor which i will replace soon enough. i am verifying this with a brand new AEM wideband installed a few months ago. most times it will idle at 14.6-15 but occasionally something makes it idle slightly more leaner.

93_Clubman wrote:There was someone else with a similar issue that ended up being caused by something different

Think this was where the CTS was also causing an a hot idle issue for Liam#18. But the fault codes you provide seem to indicate CAS & O2 sensor. Might be an idea to clear fault codes in the ECU by disconnecting the battery & pumping the brakes, if you haven't already, then drive the vehicle cold & hot, & then recheck fault codes in case there are some old ones amongst the ones got previously.

Tedman wrote:I flashed the diagnostic plug and got 3 codes, namely 3, 15 and 17.

3: No G signal
15: Oxygen sensor <0.55 V 180 sec after 1500 rpm
17: Oxygen sensor unchanged after 1500 rpm
http://www.miata.net/garage/faultcodes.html

Ref 3: No G signal:
http://www.rivercityroad.com/garage/cas.htm
http://forum.miata.net/vb/printthread.php?t=291538

Thanks for the response guys.

After doing some of my own research I checked the timing last night and found it to be + 11-12 degrees at full operating temp.
I re-adjusted it to + 14-15 degrees and while it seemed way better at the time when it was warm, but now it seems worse when it's cold!

I will re-adjust it back over the weekend when i have some time, and will look into some of the issues you guys have previously mentioned.

Plug leads and an O2 sensor are on the cards along with a full service.

Thanks guys

Try putting the timing back to standard, which is 10degrees

One thing at a time, no point advancing the timing, changing the O2 sensor, Changing leads, then still having the problem and having to work out which of the 3 changes caused it.

hks_kansei wrote:Try putting the timing back to standard, which is 10degrees

One thing at a time, no point advancing the timing, changing the O2 sensor, Changing leads, then still having the problem and having to work out which of the 3 changes caused it.

Lust set it back to 10 earlier. Engine seems less responsive and less powerful than before but that's no surprise when I retarded the timing.

Thanks mate, ordering leads and plugs now...

Update time,

Well I changed the plugs, plug leads, oil filter and also changed the O2 sensor at the same time. Then I reset the timing back to 14 degrees BTDC.

Issues still persist , doesn't seem to be going any better whatsoever

Just after checking the OBD 1 codes and got 3, 15 and 17. Absolutely no change there either!

Wondering is there 2 oxygen sensors in these cars?

The symptons you describe seem to be identical to ones that were in my car (92 NA6 85000K'S) when I bought it , it was the cold temp sensor at the back of the head , I changed it and the 02 sensor for good measure, set the timing at 10 cleaned the throttle body and reset the idle, changed the air filter, problem gone never returned and that was 12 months ago.

Basho wrote:The symptons you describe seem to be identical to ones that were in my car (92 NA6 85000K'S) when I bought it , it was the cold temp sensor at the back of the head , I changed it and the 02 sensor for good measure, set the timing at 10 cleaned the throttle body and reset the idle, changed the air filter, problem gone never returned and that was 12 months ago.

Thanks mate will check it out over the weekend, what's left of it anyway!

Gotta ghe ecu first tosee if that clears it

all sorted!

Rechecked the diagnostics, all clear!

Don't know what actually fixed it, although i still feel a hesitation but its not as bad as it was .