Ok, while my car is down and waiting on the cam bearings to show up i thought i might pick everyone's brain as to why i get a backfire through the intake, only when engine has been running for about a minute and one only once!
Engine runs beautifully before and after it! Basically, i start the car, back out of driveway, got to stop sign, pull through, go to next stop sign and then get a hesitation and a small pop through intake and off she she goes!
its gotta be something simple!, im thinking maybe its happening right as the csi stops and its just not quite warm enough! Any ideas? anyone? anyone?

As a side note, when i had all my issues with csi and thermotine being plugged into each others harnesses i did fiddle with the spring in the mass air box ( still sick and cannot think of its name), but i counted the teeth i moved it and moved it back the same amount.
Of course messing with a 30 year old spring wasn't my finest hour! but can it be possible the spring doesn't have the tension it should after my messing with it????

you guys have time to ponder this as i just ordered my cam bearings today, after they come in i get to measure and order the valve shim!! looking at two weeks before im back on the road!

 

My comment is a generic one as my expertise, if it even exists, is not on this engine!!


A pop in the intake can only come through an open intake valve.


So, when you freshen the cam bearings and clearances , it might just go away.


I've never heard of a cam gong flat in an XJ engine, but they do in others. But, this is an open issue and possibly clearance as well.


How to check for lobe wear? In an experinced engine it might be considerable.


1. A dial indicator and degree wheel. duration and openings.


2. A very careful visual. With calipers or better yet, a micrometer.
Not as accurate as 1, but could be indicative.


Carl

 

possibly just a slight lean spot in the ECU map, maybe caused by a sensor, vac leak etc.

 

Lean mixture/fuel starvation





Coolant temp sensor, perhaps. It might not be giving a 'cold engine' signal to the ECU.

The cold start injector circuit can operate only when the key is in the 'start' position.....so it's out of the picture as soon as the engine starts




Possible, but hard to confirm.....

Cheers
DD

 

Classic symptom of lean mixture.

Doug's input makes sense. I am leaning away from vacuum leak as it happens in an isolated time window while the car is in warm-up mode. However, just talking.

 

dont you wish you had an adjustable ECU like mine, slight push of a button to richen/lean fuel mixture, while you are driving, so easy!

 

I replaced all injectors, complete tune up, car was running good before but now i have a misfire and cold exhaust on on one side and normal exhaust on the other side, have a lost of power and no r p m at times

 

I replaced all injectors, complete tune up, car was running good before but now i have a misfire and cold exhaust on on one side and normal exhaust on the other side, have a lost of power and no r p m at times

 

What year, model, and engine?

Cheers
DD

 

1991 x j s classic collection coupe

 

You very possibly have a Marellli Ignition failure.

Don't drive the car !

Re-post in the XJS section and/or search archives. Tons of info.

Cheers
DD

 

I didn't replace either coil, so I'm starting with replacement of both top and bottom coil