KN Laser bad hesitation when warmed up - Please help!

Hey all,

Hope I'm in the right section of forum. I have a KN Laser 1.8l automatic, which is same as the Protege AFAIK. I have an issue with my Laser that is literally driving me insane..

When I first drive the car when cold it runs like a dream, but when the temp gets to normal operating temp, that's when it starts to hesitate and sound like it's running on 3 cylinders.

It will do this when there is very little inlet manifold pressure it seems, ie when cruising at say 70 and I just lightly put my foot on the accelerator it will jerk and cough, OR if I am at a roundabout standing still and I stomp on the throttle, it will go bababababa and then clear itself out and drive normal.

So far I have changed the following:
Plugs
Leads
Coil packs (2 of)
MAF sensor
Bosch fuel pump
Fuel filter
Throttle body to air intake pipe a couple of years ago

I have checked over all vacuum lines, brake booster lines, throttle body piping. I have checked under the car, I have checked for air leaks with a funnel/pipe. There is a very slight hiss from around the throttle body area but I can't pin point it with the funnel/pipe, it might even be a normal noise? Possible intake manifold gasket?

The funny thing is, is if I remove the temp sensor plug, it runs perfect even when warm, with no hesitation whatsoever. So hopefully this bit of info can help in fixing the issue?

I am almost at the point of giving up. It's disappointing throwing parts at it just to have it run like crap again. It actually ran worse with the new MAF sensor. But I don't want to keep buying the same new parts in chance that I got a dud.

I thought of injectors but why would it run beautiful with temp sensor unplugged?

Has anyone got any ideas? Please help as this is killing me :(

Thanks

Edit: The idle is fine too, it's only when I accelerate.
 
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Captain's log, stardate 13062024. I'm thinking it may be vacuum related because when I changed the MAF sensor it seemed to run worse, even when putting the old one back in. I just rechecked all the lines but alas couldn't find anything, but it could be the one going from the brake booster to manifold as it's got another rubber pipe over the top of it. Maybe it's cracked underneath? If that doesn't work I will take it to a mechanic and get them to do a diagnostic. It's definitely running worse now, it won't even hold idle when still. Least it will be easy to diagnose. Will report back.
 
Captain's log, stardate 17062024. I changed the spark plugs again since it was longer than I thought when I last changed them, and not to my surprise it still ran like a dog. I disconnected the battery so in the morning I'll connect it again. I think I've tried disconnecting battery in the past and letting it sit and it helped a bit resetting the computer. I do now have her booked into a decent mechanic next week so hopefully they will find something. My money is on a vacuum leak. Plugs are changed so thats one thing they don't need to check. It's fun pulling up to traffic lights with my foot on the accelerator to stop it stalling. Over and out.
 
I knew someone with an early 90's that had a crack in the plastic accordian air intake connector that would stall when the motor shifted slightly and the crack would open.
 
The funny thing is, is if I remove the temp sensor plug, it runs perfect even when warm, with no hesitation whatsoever. So hopefully this bit of info can help in fixing the issue?

I remember someone that was having a similar issue.
It turned out that the temperature sensor was covered in crap so the sensor wasn't getting a proper reading.

He removed his sensor, cleaned the corrosion off with a wire brush and that fixed the problem.
(You'd maybe just want to replace it.)
 
I knew someone with an early 90's that had a crack in the plastic accordian air intake connector that would stall when the motor shifted slightly and the crack would open.
I actually changed that big intake pipe a few years ago. I did take it off when I checked my vacuum lines and checked it for any cracks but it was fine.

I remember someone that was having a similar issue.
It turned out that the temperature sensor was covered in crap so the sensor wasn't getting a proper reading.

He removed his sensor, cleaned the corrosion off with a wire brush and that fixed the problem.
(You'd maybe just want to replace it.)
That's interesting. I might need to try that. The last couple of days it's been idling really low (enough to stall in drive) when I start it on a cold morning - BUT the babababababa running on 1 cylinder thing only happens when engine is warm.

Disconnecting the battery overnight did nothing but wasn't getting my hopes up with that. It goes in on Monday so 'hopefully' they find something.
 
Keep in mind that the ECU needs to relearn how to idle the car after the battery is disconnected.

 
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