Sputter/hesitation @ low RPMs when cold NA6

[tails]

Hi guys,

For the first few kilometres after a cold start my '92 NA6 sputters and hesitates below about 3000rpm. The idle is fine (starts at about 1200rpm and slowly drops to 850ish). When shifting into second or third it's most noticeable.

I have searched already as I understand this is a relatively common problem, so here is what I have done so far (some of these were to fix other issues);
- New leads
- Spark plugs were replaced recently (will check their gap tonight though)
- Air valve has been checked
- ISC replaced and tested
- Timing checked (currently at 13 degrees)
- O2 sensor replaced (as I was pulling a fault code)
- Checked for vacuum leaks using smoke around key lines (not gauge tested though)

Any other suggestions? I'm thinking maybe the thermosensor on the back of the engine? Though the idle behaves as it should when cold, telling me that the ECU knows it is cold. I'll have to test this at the ECU with a multimeter.

The car has no fault codes since replacing the o2 sensor.

Once the engine is warm it feels fine to me.

[tails]

Last night I checked the plugs and ECU temp sensor, both of which are fine (sensor is 0.4ish volts when warm and 2.4v when left overnight, volts decline steadily when engine is started).

I might start checking fueling issues next. Maybe fuel filter?

[droo]

If you have recently washed or sprayed water on the top of the engine, water will go into the sparkplug wells causing the cold hesitations until the car is hot.
The moisture can happen from types of weather in your area as well.

Counter intuitively, the water isn't evaporated after the engine warms up, and resumes being hesitatant for several days each morning.

Next trip you make and after the car is hot, open your bonnet, remove the spark plug leads from the spark plugs and leave to dry out for several hours.

Replug in the leads and hopefully the engine heat and lack of rubber plug blocking the top has allowed the moisture to evaporate.


The moisture affects the electrical contact of the plug lead to the spark plug and is really noticable under load (as you've described at 3000rpm)

[tails]

Leads are less than a week old and I was having the issues prior to replacing them.

I have had the wet leads issue before though (previous owner pressure washed the engine bay before handing over the keys).

[project.r.racing]

check coil packs if leads test okay.

also another question - does it use a lot of oil or blow blue smoke on startup?

[tails]

check coil packs if leads test okay.

also another question - does it use a lot of oil or blow blue smoke on startup?

Yep, I better do the coil packs.

No smoke on start up or idle. I've noticed some at WOT at night, but can't tell what colour it is. It doesn't consume much oil as far as I'm aware.

[Greeno]

I think I have this same issue in my nb8a..(bogs down, low rpm after taking off in 1st or after gear change only when cold) havent investigated it yet but ill let you know if i find the cause.

[tails]

I haven't had a chance to check the coil packs but I have run almost a full tank with some injector and fuel system cleaner added. It seems to have improved the cold running issue a little bit (it didn't really do it at all this morning). So I suspect it's a fuel related issue.

I guess it only happens when cold as it's meant to be running super rich in that state. And now that I think about it, it does feel a bit like fuel starvation.

I might replace the inline fuel filter soon to see if that helps further.

[smy0003]

If it's fuel related wouldn't the problem get significantly worse at WOT?

[StanTheMan]

I used to have the same problem to the extend that it would even drown if it stalled in the first few minutes. Once drowned it would take serious effort to start again & run real rough for a few minutes.

I did all the things you did.
spark plugs
leads
oil
O2 sensor & so on

Then I gave the engine an internal clean with some cleaner recommended by Repco. can't recall. it was in a green bottle.
I replaced the seal of the Cold air valve bypass and cleaned the inside of it with the same cleaner. I also Boiled the valve in case it was sticking. Not sure if that part of it actually made a difference or not . But its definitely not stuck now.

I then re adjusted the idle.

re adjusted the timing. Its embarrassing to admit where it was but it was wrong. lets leave it at that.
Then re adjusted the idle again just to make sure.

Then put a few tanks of 98 through it just in-case the injectors are stuck.but that's all I use now.

I also had a shitloads of dirt & crap in my air filter.

not sure which one actually made the difference. perhaps some of them together. It was over a period of about 3 months just doing bit by bit.

But it was also using a heap of fuel.

Its now running real sweet. You are just going to have to go through it bit by bit. make sure everything is good. Obviously attack the cheap things first. For the expensive bits.....see if you can loan a part before buying it.

[tails]

If it's fuel related wouldn't the problem get significantly worse at WOT?

That's what I thought initially as well, but given the warm engine naturally runs significantly more lean than a cold engine perhaps it's getting enough fuel at WOT when warm (I never run WOT when cold, so couldn't say what it's like then). But when cold, the computer thinks it's sending a bunch of fuel through (more than it would when warm) for any given amount of air, but the actual fuel being delivered is a lot less.

[smy0003]

I'd imagine that the amount of fuel used a small throttle opening when cold is still far less than WOT when hot.
I'd pull up my Adaptronic files to verify that but I don't have them on my laptop.

[tails]

That would make sense, smy0003.

Another thought that crossed my mind is that there could be moisture in the coilpack. The previous owner washed the engine bay prior to me picking up the car, and I initially had a misfiring issue but after blowing out the leads and spark tubes it was resolved (this was 6 months ago now). I don't recall blowing out the coilpack though.

I guess there could still be some in there and it condenses when cold, which would explain why it's only an issue during cold running. Will have to get somewhere with a compressor and blast out the coils.

The reason I haven't noticed this issue until recently is that I had a number of other issues related to the idle, and have been knocking them off one by one.

[speed]

I'd imagine that the amount of fuel used a small throttle opening when cold is still far less than WOT when hot.
I'd pull up my Adaptronic files to verify that but I don't have them on my laptop.

Correct

[droo]

Have you atleast tried drying out the plug wells in the head?