1999 Honda civic LX misfire/stumble at 70-80mph.

Hello, I got a 1999 honda civic lx with a misfire/stumble that really only occurs at 70-80mph. it will most of the time give random misfire codes or cylinders 1-4 misfire codes occasionally. My short term fuel trim on the Autel reads 1.10-1.20 at 70 to 80 mph. it takes about 10-15 mins of driving for the problem to occur. car idles great but short term fuel trim is 1.03-1.05. short term continues to climb positive when given gas.

checked fuel pressure at idle its around 35psi. with the regulator unplugged it goes up to about 40-45psi. it holds pressure when doing a static fuel pressure test. blew out all the fuel lines with compressed air, no contamination. Fuel pump, injectors, fuel filter and fuel pressure regulator have all been replaced. checked fuel injector connectors with noid light. oxygen sensors. distributor, wires(NGK wires) and plugs have also been replaced. checked compression and performed leak down test. map sensor has been replaced as well. adjusted valves. sprayed for vacuum leaks.

I did notice a hairline crack in the cat by the oxygen sensor but i dont see any exhaust marks coming out of the cracks. so i dont think its all the way through. so, i dont think its making my trims go lean. correct me if i am wrong though.

Thanks!
Any help is appreciated!

My first thought is a coil issue. Do you have a way to look at primary and secondary ignition?

distributor was replaced. so, the coils would be new. the problem still followed after distributor replacement. i can look at my scanner but i dont recall any function for viewing primary and secondary ignition.

According to Alldata, your fuel pressure is low.

With Regulator Vacuum Hose Disconnected
47-54 Psi.

Well looks like I should get a new fuel pump coming. I am gonna check voltage to the pump just in case. When I do order the new pump, what are the best replacement brands?
Thanks!

I looked up the fuel pressure specs on all data for a 1999 Honda Civic LX and it has 38-46 psi for vacuum hose disconnected. I was driving it around last night and fuel pressure with wide open throttle was at like 45psi

:blush: My apologies. I was looking at a Honda Accord LX.

Nice catch.

No worries, I just appreciate the help! Any other ideas of where to look? I think the fuel pressure is alright. I took vacuum reading off the manifold the other night and my gauge was bouncing rapidly back and forth between 16in.hg and 20in.hg at idle. Is this a cause for concern? Do you have any good resources for the different meanings of gauge readings?