M42 stumble and hesitation chasing

[rojeho]

M42 that idles pretty smooth but sometimes hesitates real bad on acceleration. Sometimes accompanied by muffled pops out the exhaust, sometimes it just goes away, other times it goes away with a surge of revs and power.

No codes, new plugs, coils all check within resistance spec and close to each other, cleaned the ICV, MAF and very carefully checked the intake rubber boot. After doing this, the problem got much better but is still there. Instead of doing it once every 5-10 acceleration pulls, it's once every 30-50. I thought that was odd and that I either would have fixed it or not.

What's next? Plug wires? O2 sensor? Something else?

It's so close to running smooth.......

[randerson920]

When you took of the upper intake manifold to clean the ICV, did you check ALL the vacuum lines? Not just the intake boot? When I did this I found that every single vacuum line was cracked and brittle, replaced them and noticed a great increase in power. (as much as an m42 will allow) You can quick test by starting you engine and spraying some throttle body cleaner ($2.50) around your vacuum lines and checking if you engine hesitates. If it does, that is the cleaner being sucked into the intake via the vacuum lines, replace where needed :)

[fast84gti]

This may not apply to your engine, but check the vacuum lines on the injectors. I think 1993 was the first year to have air shrouded injectors, but I am not sure. My lines were all broken and messed up. 80 dollar fix with part only available at dealer :(
If you don't have the injector vacuum lines, then remove injectors and replace the o-rings. I didn't get my orings from the dealer. They are expensive as hell. I went to a store called "O-rings and things" and paid a dollar for all 8. They were gas rated, too :D Just bring an old oring with you to match for diameter and thickness...

*I would replace the oxygen sensor if you don't know exactly how old it is... Then you know it's one less thing wrong...

[rojeho]

I probably didn't check the hoses enough. In general it's in very good shape, 110k and the hoses look good, but I should inspect them closer.

Based on the extra wire zip tied, the O2 sensor is not original, but I don't know when it was changed.