k1e1v1i1n

Hey guys I do have a small crack in my header and I do have a replacement sitting on the table next to me but I am not sure this is 100% the issue. is there anything I can do or look at to see what is going on. maybe my O2 sensor is no good ? Car seems to run fine super smooth everywhere over 1k but when ideling around 800-900 it can stumble a little.

harveyf

I would be skeptical that a cracked header would have any noticeable effect on mixture. There are so many potential causes for a stumble at idle. Spend some time reading previous threads. Search on stumble, idle miss, etc.

k1e1v1i1n

Thanks I was thinking the same about the crack I am guessing maybe the O2 sensor I will have to poke around a bit

jhowell371

Possible causes are the O2 sensor or the fuel pressure regulator or dampener diaphragms maybe ruptured allowing excess fuel to be sucked into the intake.

harveyf

BTW are you basing your conclusion of it being rich solely on the stumble at idle or are you seeing something out the tailpipe. After it's warmed up, if you hold a paper towel or white cloth across the tailpipe airstream, are you getting black soot?

k1e1v1i1n

its the smell if you drive behind me its pretty strong.

well that and 400 miles to a tank

divil

If you unplug the O2 sensor the car should run pretty close to the proper AFR once warmed up, assuming nothing else is wrong.

k1e1v1i1n

I will look into that

harveyf

It might be a good idea to mention the year/model of your car, because what I'm getting ready to say may or may not apply.

I learned from reading Clarks-Garage that the fuel pressure regulator, at idle, reduces the fuel pressure. This in effect reduces the fuel dose per injector cycle and leans out the mixture. Don't quote me exactly on the numbers but I believe it takes full system pressure (44 psi?) and reduces it down to 36-38 psi at idle. It does this based on a vacuum signal just downstream of the throttle. This is the small vacuum line that goes to a Y connection that leads to the fuel pressure regulator and the fuel pressure damper. If this vacuum signal is "corrupted" then the fuel pressure will remain high and the idle will be rich.

2 things to check- Get a vacuum gauge, tee into the line I described, and confirm you have decent (> -15 inches) of vacuum in the line with throttle closed at idle. Even better is to get a fuel gauge that mounts on the end of the fuel rail and observe the fuel pressure reduces per spec at idle.

k1e1v1i1n

I have an 86 but the engine is from an earlier year.

harveyf

In that case, the above info is applicable.

mazdaverx7

How old are the plugs, wires, and cap? Does the car still have the original catalytic converter? How is the air filter? Just some basics before trying to diagnose something that may not be there.

Noahs944

Unclean grounds can cause rich mix

k1e1v1i1n

cap and rotor are brand new wires and plugs are not I currently have a hot air intake I need the brackets to install the stock air box. I am sure the cat original and I would think if the ground was unclean there would be a whole host of other issues.

Ed Petry

Depending on the circuitry in the DME the grounding of the idle and running staged could be different. I can’t say for sure that this is an issue with our cars but grounds are always a great place to start troubleshooting.
Good luck.