Slight Misfire /?Fluctuating Revs At Light Throttle??

[Steve 940]

Hi,

940 2.0 Turbo, get an occasional issue of either slight misfire / revs fluctuating or hunting between 2k - 2.25k rev counter needle also gently fluctuates when running on dual carriageway around 50-60 mph on a light throttle. Only happens around once or twice over say a 20 mile run.

Seemingly happens at random, and if you boot the throttle it revs out no problem and it disappears, if you ease off, then again the problem disappears, and I can accelerate back to cruising.

I had half been hoping it was my faulty in tank pump issue recently replaced, it's sorted the noisy main pump issue, but occasional hesitancy remains.

Just wondered if anyone had any pointers on where to start, I am wondering if it could be a fuelling or ignition problem, I do have a new main fuel pump and filter, and new plugs/rotor arm that I have to do a full service when I get a moment.

Unfortunately I don't have a fuel pressure gauge as I understand this can be checked at the valve on the fuel rail.

Appreciate any pointers.

Cheers
Steve

[Laird Scooby]

First thing i'd check is plug gaps Steve, should be 0.65-0.7mm, most new plugs (certainly NGK) come factory set at 0.9mm so if your plugs are old and weren't gapped to the corect setting, then the spark is trying to jump further than it's designed to. Cruising is the leanest mixture possible so could easily be a plug breaking down under load if the gap is too wide. Also worth checking the fuel pressure regulator isn't leaking fuel past the diaphragm, with the engine idling pull the small bore hose off the FPR and put your thumb over it, there shouldn't be any fuel coming out of the stub on the FPR. If there is, the FPR is in need of renewal.

[Steve 940]

Hi Dave!

Many thanks for pointers, when I bought it some 8 months ago it was supposed to be 'serviced', so I had assumed perhaps incorrectly, that plugs would have been changed. Part of the deal was timing belt change and front seals as there was a leak on the timing cover, and they did supply invoice from Brookhouse for those parts. Of course if new plugs were fitted, no guarantee they gapped them correctly.

As you know I had been baulkng at doing my in tank pump, and had done the 'sniff test' on the small hose on the fuel regulator, could not smell any sign of petrol. Although that was with engine switched off, will do the test again with it running.

As you've probably gathered I'm a bit slow getting upto speed on this spanner wielding, the last plugs I changed must have been 30+ years ago, do I go airline to blast dust etc. from the recess before taking old ones out, copper grease (circa 1980's) sparingly applied on new plug threads?? Obviously taking care of not to cross thread.

HT leads are marked Bougicord, so I think these probably are original, or not??

Cheers
Steve

[Laird Scooby]

Originally Posted by Steve 940

Hi Dave!

Many thanks for pointers, when I bought it some 8 months ago it was supposed to be 'serviced', so I had assumed perhaps incorrectly, that plugs would have been changed. Part of the deal was timing belt change and front seals as there was a leak on the timing cover, and they did supply invoice from Brookhouse for those parts. Of course if new plugs were fitted, no guarantee they gapped them correctly.

As you know I had been baulkng at doing my in tank pump, and had done the 'sniff test' on the small hose on the fuel regulator, could not smell any sign of petrol. Although that was with engine switched off, will do the test again with it running.

As you've probably gathered I'm a bit slow getting upto speed on this spanner wielding, the last plugs I changed must have been 30+ years ago, do I go airline to blast dust etc. from the recess before taking old ones out, copper grease (circa 1980's) sparingly applied on new plug threads?? Obviously taking care of not to cross thread.

HT leads are marked Bougicord, so I think these probably are original, or not??

Cheers
Steve

Many garages view an oil/filter change as a "full service" Steve so worth your time going over everything that should have been done and checking it.

Probably no real need on a red block to get the airline out and blow the plugs off before removal unless there is obvious debris around them - would be unusual though.

As for a smear of copper grease on the threads, yes, spot on. It's possible they used Champion plugs in it, if so just replace with NGK plugs and that will probably fix the problem. It's also possible they didn't even bother looking at the plugs too!

Bougicord were OE kit and the recommended replacement as they have a long service life and do the job very well. They can be obtained from PFV but are listed as "Bosch or Bougicord" and i believe you have to specify you want Bougicord.

https://www.partsforvolvosonline.com/

One thing you will need when you replace the plugs is a short length of rubber hose, either 1/4"/6mm bore or 3/8"/10mm bore. Fit the length of hose onto the plug lead end of the plug and use it to screw the plug in - if the plug cross-threads the hose will slip and the plug can be removed and refitted correctly - #4 plug is particularly tricky as it is a backwards and up angle - you'll understand that description on #4 plug when you get to it. Plugs #1-#3 are relatively easy to get to but the hose method of fitting is still recommended.
The smaller bore hose fits the nipple on the end of the plug that the plug lead connects to electrically, the larger bore hose fits the ceramic insulator so either can be used.

If memory serves the spec in the HBoF (Haynes Book of Fantasy) says 0.7-0.8mm plug gap, however that would have been written when V-groove plugs weren't common, setting them to 0.65-0.7mm takes the V-groove into account and also allows for some peoples techniques with feeler gauges being looser than others.

The test on the FPR needs to be done running or the fuel pump won't be supplying pressure to the FPR and the diaphragm inside it so a pinhole won't give the tell-tale dribble of petrol. Worth noting that the FPR springs can go weak and cause the same problem, usually when it does it causes a slight hunt at idle, particularly when using the "creep" of the auto for parking.

There may be other causes of the hesitancy while cruising but let's go for the simple stuff first!

[360beast]

I would NOT put any grease on the spark plug threads, NGK themselves tell you not to.

https://ngksparkplugs.com/en/resourc...ut-spark-plugs

Pretty sure I bought the last or last but one set of Bougicord HT leads from pfv and they said their supplier can't get the raw materials for them so they have no idea when they will be back in stock.

If you change your plugs when the engine is cold then you can start them by hand, #1 I usually use an extension and a UJ to get started as the turbo intake pipe is slightly in the way, just don't go at it with the ratchet until you know it is threaded correctly.

Plug gap in all the manuals states 0.7mm for plugs so set it to 0.7mm, you will need a set of feeler gauges.

What fuel are you using?

Have you removed the distributor cap to check the condition of the contact points and the rotor arm? If you haven't resealed your distributor then you may have an oil leak and if it is a bad one then oil will be over the contact points like mine was, I resealed the distributor and all of my 940's hesitation was cured.

I would also for good measure remove the ignition module off of the heat sink, clean it up and apply new heat sink compound to the back of it.

https://www.amazon.co.uk/Servisol-31...c=1&th=1&psc=1

[TonyS9]

Does it seem like the engine is dropping in rpms or just power. ie the rpms drop more than the car speed? Manual?

I'm thinking the ignition amp. You will lose spark at certain temperatures, it can be quite intermittemt.

Otherwise a good ignition service may well help, if its just general dirt or oil.

I think a fuel issue would be more consistent.

A weak spark (from say a wet distributor) can be drowned out by extra fuel so if you floor it it bogs down and can cut out. More gentle accelerator can get the rpms up and past the problem area. But it doesn't seem like that, that also why I'm thinking ignition amp.

[Laird Scooby]

Back when the 940 was made 30 ish years ago, NGk and most other spark plug manufacturers used to say to always use anti-sieze compound. Spark plug technology may have moved on (including the V-groove electrodes) but the head is still the same as it was in the 70s, 80s and 90s - as such i would go with the major part (the head) and still use anti-sieze compound. Then fit the plugs using the time-honoured nip it up finger tight then 1/2-3/4 turn with the plug spanner.

If the supply of Bougicord leads has been exhausted in the UK, i'd go for Denso instead :

https://www.rockauto.com/en/catalog/...plug+wire,7224

Cleaning and re-applying heatsink compound to the ignition amp was part of the next stage in my process. If you decide to do it now, when you clean it do NOT use anything abrasive, just use solvent cleaners and finally a fine metal polish like Solvol-Autosol to get the heatsink and back of the ignition amp to a smooth, shiny surface, cleaning off any residue of polish afterwards before applying heatsink compound. Also worth checking the earth for the ignition amp, easily found by following the loom from the plug to the nearest earth point, usually about 6-8" aft of the plug as a bad earth there will reduce the output. This will be most noticeable under the most difficult firing conditions, i.e. under cruise conditions as a weak mixture is the hardest to fire.

[Forrest]

The crux of the NGK advice not to use anti-seize compounds on the spark plug threads seems to be that it increases the risk of over-torqueing them. If you adjust the torque accordingly this should not cause a problem. I usually go for 30% less than the recommendation for dry threads, or do as the Laird suggests and don't use a torque wrench. Personally I use ceramic grease on plug threads.

[Laird Scooby]

Originally Posted by Forrest

The crux of the NGK advice not to use anti-seize compounds on the spark plug threads seems to be that it increases the risk of over-torqueing them. If you adjust the torque accordingly this should not cause a problem. I usually go for 30% less than the recommendation for dry threads, or do as the Laird suggests and don't use a torque wrench. Personally I use ceramic grease on plug threads.

I totally agree with that but the torque recommendations for the red block (25lbft rings a bell, might be different and i can't be bothered to double-check) are for plugs with some anti-sieze compound but if you're going for 30% less and they're staying tight, i wouldn't worry. The main reason a torque figure is applied to the plugs is to help prevent siezure in the threadsafter 12k miles and not fall out in between. Granted the threads are fine (compared to normal M14 threads or M10 threads as applicable) so will be inherently weaker than coarser threads (and also easier to cross-thread) so reducing the torque setting is probably a good idea on high mileage engines anyway.

My other argument about what NGK are saying about not applying anti-sieze to the plug threads is they supply Lambda sensors with anti-sieze already on the threads, presumably the threads on those are the same trivalent coating they claim the plugs have so which is correct?
Ultimately i'd go for experience over new learning so a smear of copper slip would be what i'd do. It's not failed me in 35+ years so no reason to believe it's going to cause problems now!

[Steve 940]

Hi,

Many thanks to all the contributors on this issue, most informative and some great tips that I am sure will no doubt be needed, I think from all this input, it is clear that first port of call is to do a proper full service, plugs I have are genuine Volvo and rotor arm is I believe pattern, both from Brookhouse.

I think a few evenings of trawling the FAQ section on all these topics are in order whilst the bits get here.

Will need to source leads and distributor cap, and will see what I can find, PFV have Genuine Volvo cap, but genuine leads are, as you guys have mentioned, no longer available. Will check out Rockauto.

Fuel I usually use is I'm afraid E10, and I normally fill up at our local Co Op, Tescos at a push, and if I'm passing a Shell, I do occasionally put in the higher grade, albeit more pricey.

Stupid question perhaps, if I order HT lead set, do these normally come with a new coil to cap lead as well? If not, should this be replaced as well??

Would appreciate some further guidance from your experience in ordering parts.

Need to get HT lead set, and distributor cap, FCP Euro in US do apparently have Bougicord leads at around $50 or genuine Volvo at $90?? don't think USA got the 2.0 Turbo, is there any difference and would either of these fit?? What cap is best?? looks to be quite a difference in price from around £20 for pattern from PFV up to FCP Euro at $80 for Bosch or $120 for Genuine Volvo.

Obviously a bit wary of ordering from US as to igettong the right ones, also duty and post to consider.

https://www.fcpeuro.com/Volvo-parts/...&b=6&d=42&v=11

Cheers
Steve