Basics.
I’ve had a 1991 civic dx hatchback with the dpfi and 5spd manual for the last 3-4 years and up to about 450,000miles.

all parts I’ve replaced over the time.

distributor about 3 years old.

Ignition coil abt a year

sparkplugs 3 months ngk’s

distributor cap and rotor less than 6 months

spark plug wires less than a year ngk as well.

Alternator 2 years old

coolant temp sensor for ecu (two wire)Oem Honda 6 months and plugin for that sensor

Had to do a new exhaust valve because on cylinders number 4 had a crack and small hole. So all gaskets were replaced obviously as well. About 4 months ago.

fuel tank and fuel filter less than 8 months old.

main relay less than 6 months old

02 sensor less than 1 year old

does have 4-2-1 header I got for free. Been on for a year or so. And also a test pipe replaced the catalytic converter. The 02 sensor is placed right before it now.

I might be missing some items but that’s what I can remember off the top of my head.

so I’ve had a problem for the last year and a half approximately. When it first started it was subtle.
basically when the car got up to temperature. (when it goes into closed loop). It would start jerking and bucking under about 3/4s throttle at any speed , And it was subtle and intermittent at first. But then gotten progressively worse over time.

And my way to always get around it was to unhook the coolant temp sensor for the ecu and it would go into open loop and run perfectly besides a slightly high idle and run a little rich. And I did this so I could drive it still as my daily until I could figure out the issue.
But now it’s starting to do it at high RPM now even with it unhooked and it’s very strange. It’s like a some what consistent “buck” (and what I mean is it feels like it like looses fuel pressure or something like that feeling when it bucks ) “loss of power” about every 1-2 seconds. And in neutral it does the same thing constantly.

things I have tested.

tps sensor was .5v at closed throttle
Wot voltage was 4.35 and through out the range looked perfectly smooth.

i have cleaned the the iac and it seems to function perfectly

pvc valve on intake runner works and is clean

Fuel pressure is perfect

distributor and engine timing is set perfectly

I have cleaned the primary injector about a year ago and tested it some what with a battery.

cleaned intake runners and throttle plates

I’ve checked and all 4 cylinders are getting perfect spark to the spark plugs

compression is great for the mileage also 180s-190s

No codes are being shown beside the code 6 for the coolant temp sensor that I usually unplugged.
And just some more information on that. I usually hook it back up in the cold mornings to get it started and then unhook it to drive it.

I’m just running out of other things to check and replace.

I’m sure I’m missing some information. But, I’m hoping some one out here has had a similar thing happen. I love this little car and I wanna drive it as long as possible. Any advice is appreciated. And if you made it this far, I love you.

 

First check for vacuum leaks as well as vacuum line routing. Pretty common for them to crack and worth while to just replace them all. If you have any vacuum caps that are blocking off ports, the rubber ones from the parts store seem to have a very short lifetime. Second, moving to a header results in more heat loss. The one wire O2 sensors require engine exhaust heat to get them to operating temp which a header can cause them to read incorrectly. The easy fix is to wrap the header in heat wrap and the less easy fix is to move to four wire O2 sensors that have a heating element built in. This requires a dedicated relay but not rocket science to figure out. Last, double check the wiring for the TPS sensor. The DPFI plug is wired different than the MPFI. Make sure it is wired correctly as well as the TPS is sending signal out the correct pins.