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[humbucker]

hi all

just replaced the head gaskets and all associated parts due to rotten gaskets and ongoing leak problem. this is now cured. i am happy.

however, when i got the car back i note that it is slightly lumpy, especially around the 50mph mark. also, tickover isnt great with the needle running between 250 and 750 rpm up and down whereas before the work was done there used to be a quiet 500rpm constant.

also, there is almost some kind of air circulation noise from plenum, not something that sounds like its leaking as such, but something that wasn't there before.

when job was being done garage said HT leads could do with replacing on one bank as one looking a little worse for wear, as they were experiencing slight misfire. i have never had any problem in this dept. sparks were changed within the last 8 months with dealer supplied GM parts. breathers cleaned last weekend.

from scouring the forum and other sources i am led to believe possible airflow problem? can anyone advise?

car is 3.0 v6 elite.

thanks!  

[jjleonard]

I'd definitely check all the vacumn pipes and air connections to the plenum - even making sure that the plenum is seated properly and the plenum to air intake / breather box seals are good.

Air leaks around the plenum can definitely cause lumpy idling and misfiring as the fuel / air mixture is all over the place.  

[humbucker]

thanks for the advice. i'm right in thinking plenum should be silent right? i dont recall any noise from it before and ive removed and replaced it many times when working on the engine.

[Entwood]

To check for vac leaks, get some aerosol brake/clutch cleaner and spray lightly around the suspect area whilst the engine is at tickover, as the cleaner is drawn into the system and burns the engine note changes for a few seconds, thus the area can easily be narrowed down quite accurately. Once completed a quick wipe down with some papertowel and not only have you found the fault but you have a nice clean engine ..  

[jjleonard]

thanks for the advice. i'm right in thinking plenum should be silent right? i dont recall any noise from it before and ive removed and replaced it many times when working on the engine.

Definitely - the plenum should be as silent as possible (i.e. you shouldn't be able to hear any air noises over that of the engine!)

Use Entwoods' tip to check for air leaks - wish i'd realised that when trying to find my leaks!

[humbucker]

its producing a slight chug/pop noise out of rear end at the same time while its ticking over. could this still be caused by airflow / vacuum probs?

[Marks DTM Calib]

Yes...but also check the plug leads are connected correctly to the DIS pack on the 2-4-6 bank (common mistake)

[humbucker]

as ever, thanks for the advice. i'm going to drop the car back at the workshop tonight so i'll leave a little note asking them to check the leads and the vacuum pipes. hopefully that will sort it, i had no problem before it went in so i can only assume its something dislodged when the engine's been reassembled. i'm hoping its not the ht leads but the sound from the plenum leads (ho ho ho) me to believe its an airflow issue. i hope... anything else i could ask em to check that might cause it?

incidentally, i take it that it isn't obvious which way back the leads go into the DIS pack. is this something you (i) could tell by looking?

[Marks DTM Calib]

Like I say...lead connections to the DIS.....th 2-4-6 bank dont connect up as 2-4-6.....its more like 4-6-2 and hence if not thought about can easily be done wrong!

[humbucker]

'k thanks for the heads up, i'll report back. i'll be leaving the motor with them tonight for them to check over while i'm at work tomorrow so will see what happens when i pick her up...

[humbucker]

just a thought, but would dodgy ht leads cause erratic idling, or only misfire on acceleration?

[Marks DTM Calib]

It depends how dodgy......when juts slightly going they tend more to cause issues during load

[humbucker]

ok thanks for that, probably vac then.

 I checked the car when it was running cold and when warm and had a good listen around. The plenum should be silent but it is now making a fairly audible noise like air circulation, which you can hear over the sound of the engine. When I listened to the back of the engine bay behind the plenum I thought I could hear a sound similar to that of a compromised vacuum pipe, a sort of hissing / blowing, but was unable to get behind there while in the street.

This might explain the lumpy idle and acceleration. Will ask workshop to double check the vacuum pipes and that they are connected correctly. I cleaned them all out last weekend so shouldn’t be blocked up.

[humbucker]

right, i'm going to drop the car off now, will report back when i hear anything. thanks for all your advice and help, hopefully will resolve this tomorrow  

[humbucker]

hi all

rather than muck about trying to work out what might be causing vac leak issue or unseated gasket etc the workshop stripped the engine back down to the oil cooler and replaced all the gaskets and o rings AGAIN with a brand new set, checking and building back up again as they went. the result of this is that the air circulation noise has dropped considerably. this suggests that there was probably a compromised gasket or something somewhere.

however, the misfire/jolt is still present. workshop suggested passengers side bank of HT leads looking pretty bad and should be replaced. symptoms as follows:

idling is fairly jerky between 500 - 750rpm, particularly when warm. accelerating is fine until you get to between 50 - 60mph when she seems to misfire or jerk quite a bit. past 60 mph seems to be fine. under 50 seems to be fine. if i drop the cruise control off or decelerate at any speed she runs fine, its mainly when accelerating or maintaining a speed circa 50-60mph.

does this point to HT lead issue? i certainly havent replaced them since ive had the car and that was 83k miles ago. plugs were changed a few months back, breathers cleaned couple of weeks ago. and if it is the HT leads, what is the recommended procedure ie. GM parts only?

thanks as always