Hello! First time in here, I've looked far and wide but I was never able to pinpoint exactly what my problem is, so I'm finally asking here.

I own a 90 (or 91, not quite sure) USDM Civic Si, I believe the engine to be a D16A6 (it has about 160k miles on it) but again, not quite sure because I live in the EU and the car is, from what I've been told by the previous owner, an import from the US.

It has a very specific issue that I'll try to describe here :

When I first start it up for the day, even if it's really cold out, it'll start up perfectly fine in a couple of cranks and run just as well, and it'll keep running just fine with no issues. I would then park it, go grocery shopping or whatever, and when I come back and try to start it up again it just runs extremely rough, even struggling to start sometimes. It struggles to keep a stable idle, reacts extremely badly to sudden throttle inputs (often just dropping the idle to near 0 or straight up stalling) and makes driving the car in the lower RPM ranges really difficult. The problem seems to completely go away in the higher RPM ranges, like I can go on the highway just fine and it'll be happy there cruising around the 3k RPM range, and after a while of driving at higher RPMs it seems to stop having issues altogether (if I have to take a highway trip for a couple of hours, the car seems fine again when I arrive).

It seems to exclusively do this in cold weather aswell, it did it last year around the same time and through all of winter but it stopped completely as soon as the weather warmed up. Because of this my main suspect right now is the engine coolant temp sensor, I've got a replacement on hand but I'd like to get a second opinion before committing to beginning work on the car, since I don't have much space available.

The car doesn't really have any other issues apart from burning oil, the rate at which it's burning it is pretty high but it's still within manufacturer's specs so I've not been worrying about it too much, this drivetrain does have a lot of miles on it afterall.

Presumably coil wires + spark plugs are fine aswell but haven't checked, my troubleshooting didn't really point me there since it runs perfectly fine aside from that one specific case. Distributor was checked by my mechanic last winter aswell and he found no issue, so he just cleaned the cap but that didn't fix the issue. For what it's worth (tho I'd say he's more than trustworthy) he also told me last time I visited (a few months ago) that it ran just fine and he noticed no issues whatsoever.

That's about all I have but if there's anything I can clear up so it's easier to troubleshoot feel free to ask! Thanks in advance

 

Coolant temp sensor could definitely be the cause. Is it throwing any codes by chance?

 

I've not checked for codes yet, will give that a shot tomorrow if I can!

 

did you end up finding a solution to this? I also have a 90 Civic SI (made in Canada not US fwiw) and the first time I start it each day it's super smooth, but it won't start if the engine is already warm. I have only had the car for a week and it's been doing this since day 1 so I'm not sure if it is a winter-specific issue or not, but I too am suspecting the temperature sensor. Did you replace it? Curious to know what happened.

 

Hello! Sorry I kinda forgot about this post, but yes I did end up replacing the engine coolant temp sensor! It did seem to make the situation better and for a while it was completely fine, but a few days ago it started struggling a bit again (in the same conditions i described in my initial post). However it wasn't nearly as bad as before so I suspect it atleast did something good? I also noticed I was losing coolant so i'm suspecting it might've been a slight overheating issue aswell, my radiator drain plug was leaking so I tightened that up and I'll be checking to see how it evolves in the next few days

I'm also going to replace spark plugs and check wires next. I'd be curious to hear an update if you do end up replacing the sensor aswell

 

Thanks for the update, this is actually really interesting because I just noticed on Sunday that I am also loosing coolant! Unfortunately I have yet to find the source of my coolant leak – no drips of coolant anywhere, no sign of it in the oil either. I have a cardboard box under the car just in case, and even tried looking with a UV light and I'm not seeing anything. So I'm going to try to solve that mystery before I replace the temp sensor just in case the coolant leak is the issue. Going to do some further testing of the temp sensor with a digital multimeter as well. If all of that fails me I will also look to the spark plugs and maybe the EACV. This car is teaching me a lot lol.

Would love another update if you end up finding the solution. Good luck.

 

Bleed your coolant. You got a new sensor and it worked better but there is air in the system from the loose radiator drain plug?
Other things to check main relay, eacv, fitv. Plugs and wires probably don't have anything to do with it but nice to do.

 

I replaced my temp sensor and my car starts so much easier when cold, and actually restarts when hot. I've uncovered some other issues that will need work before I drive it again like stalling while idling at stop signs. My next fix will be the a coolant leak, so like you that could be impacting the temperature of the system. My upper rad hose was replaced a few years ago (before I owned the car) and I think it's time to replace the lower one since it was shooting coolant today at the end of a test drive.

 

update: I fixed the coolant leak and bled the air from the system and that seemed to resolve my issue! Surely there will be more issues to come with this old car but I’m stoked to have her running again!