Odd occasional hesitation under acceleration/boost
 

Hi all,
I have a slightly odd stumble/hesitation under power in my 940. I'm midway through doing a "stage zero" on it (have changed plugs, oil and filter, PCV, cleaned IACV, throttle body and flame trap, new cat-back exhaust, checked all vacuum hoses, removed disintegrating foam in air filter box and have secured the lid with a luggage strap as all the clips are broken).

The issue seems to be between 2k-3.5k in the rev range when pulling hard in a high gear. If I change down and clog it, it seems to clear at higher revs.

I have a new distributor cap and rotor on the way along with new Bosch HT leads, so hopefully that will help. The old plugs indicated a slightly lean condition so I also have a new fuel filter to fit as a precaution.

The car has only done 75k miles and runs very sweetly other than this stumbling. Am I missing anything obvious?

Plug gap, timing belt tension, dirty injectors - those are my first thoughts. Did it do it when you first picked it up at all? What were the plug gaps on the old plugs and what have you set the new ones to?
Could also be an HT lead breaking down under load, dirty coil tower, cracks/dirt on the dizzy cap and/or rotor arm - there are many possibilities.

The engine speed range you've quoted is right on the resonant frequency for most ignition coils when they give their highest output so as the plugs will be harder to fire under load, the spark is likely to look for an alternative route and cause a misfire in the rev range you're getting it.

Might pay to wait until you've got the new leads etc and have fitted those, you may uncover something as you do them and that may well cure the flat spot/misfire.

I had that when I first got mine. New set of Bourgicord leads and hey presto. The Bosch leads that were on the car fell to pieces

Thanks gents!

I think I'll start at the engine end of the ignition system and work backwards. The car did have a bit of an occasional miss when I was on the way home from picking it up, but it didn't seem as noticeable as it is now. Having said that, I've done quite of lot of other stuff to bring the car up to snuff, so it may be the case that the performance originally was all a bit under par, so the miss was less noticeable.

I had a spare generic ignition amplifier kicking about so plugged that in this evening and took it for a drive...no different to the Bosch amp that's fitted to the car, so that's one thing to discount. I also removed the fuel pump relay and reflowed the solder joints, but it's a recent Professional Parts unit so I don't think that is the cause of the probs, and the reflowing made no difference.

In the rev range I mention it still misses at WOT, so I think I can discount the Lambda sensor as I gather that runs open circuit at WOT so is effectively out of the loop.

So, I think my working list will be:

New leads, dizzy cap, rotor arm and o-rings
If still no joy, new coil
Fuel filter (which I have ready and plan to do tomorrow)
If STILL no joy, check fuel pressure at rail.

It COULD be a bunged-up cat of course, but I'm not convinced by that theory. The cat is the next exhaust component that I need to replace as it's a bit crusty, so if that ends up being a suspect factor I could gut it and refit to test the theory before replacing it...

Plugs wise, the ones I took out had a pretty hefty gap and worn electrodes. My guess was that they had been in there for 20k+ miles...I fitted my usual NGK BPR6ES, gapped to 0.7 as per all the other Redblocks I've owned, the only potential issue is that the bag that ECP gave me with my parts in split in the car park, and two plugs (in their boxes) hit the deck before I could catch them. I was concerned, but couldn't find any insulator damage so went ahead and fitted them.

I suppose, on the outside, there is a chance that a hairline crack in one of the insulators is opening up when the engine is gets hot, and causing an arc to earth when under load...I believe I have some spare plugs somewhere so will switch them over tomorrow and see what gives.

I think you may have found your problem Phil - doesn't take much to cause a problem in a plug!
Even if you re-gap the originals after cleaning them you'd probably be better off than running with 2 suspect plugs.

Worst case scenario i'm sure ECP would change those plugs if you told them they misfired and you proved it by refitting your originals with oodles of miles on!

Good plan! I believe there is a fresh set of NGKs in one of the rear storage cubbyholes that I didn't know were there when I bought the set from ECP, so will investigate those and change them on the morrow.

I keep reading about plug options, copper core Vs standard, grooved electrode Vs solid, resistor Vs non-resistor and so forth, I've always used BPR6ES but am tempted to try the BCPR6E grooved copper cores sometime. What are your preference?

There's not a wild amount of difference Phil, i prefer the BCPR6E(S) over the basic BPR6E(S) as i feel the copper core gives a better spark.

However i've no evidence to support that other than gut feeling and advertising blurb so i generally go with whichever one i can bulk buy a set of 6 more cheaply.

Usually that happens to be the BCPR6E(S), the S denotes a short thread if memory serves but either the normal or short thread work equally well in both my beasts without any problems and it's only 2 or 3 threads shorter anyway!

Normally i get a set of 6 for about a tenner, delivered (or collected if i put the hard word on my local motor factors) and at that sort of price the thread length gets less important! :tounge_smile:

As for the multiple earth electrode jobbies like the Bosch Super-4 or whatever they call it these days, for a red block or either of the two i've got, it doesn't offer any practical benefit, same goes for the Iridium plugs and other, "high-falutin' type plugs" that, for the most part, are just an expensive option to the basic plug which will do the same job in our beasts.

As a rule of thumb, unless the engine has been heavily modified, the manufacturers recommended plug is usually the best to go for, whether you go for the same make (i think Volvo specified Beru originally and then Bosch) or a reputable equivalent such as NGK, Denso or similar, as long as the spec is the same.

There are arguments that the V-Groove plug improve the spark and flame propogation and to be fair, there is something in this.

Whether it translates into improved economy, power, response and emissions is another matter though.

The V-Groove creates two spark paths from the centre electrode and the theory is it then creates a small plasma ball which extends the burn time of the spark.

You can see this in a practical way with this lunatic playing with high voltage :

https://youtu.be/lT3vGaOLWqE

His language is a bit fruity at times so those of a nervous disposition should retire and take Horlicks instead of watching it but it demonstrates how plasma discharge is generated.

Enjoy! :tounge_smile:

ahem...

https://www.alfabb.com/attachments/ngk_sym-gif.48318/

Thanks Ash - been trying to find that for quite a while, seems to have disappeared from the web version of NGKs catalogue or has been put into a part of it i can't locate! :thumbs_up: