Engine 850: B5252S/2.4/10v: Missing/Hesitation Under Load (again, ideas?)

[DmcL]

right.. so i thought i had the problem cured with a new distributor cap and rotor arm but coming home from work today it started missing occasionally. nowhere near as severe as previously and so far only happening under load in the lower half of the RPM range also no intake/exhaust backfires as before, just some on/off type hesitation.

it doesnt happen in 1st gear, the wheels just spin but if i try in 2nd or 3rd with the revs between 1-3k RPMs it will miss then go as normal for a second or 2 then miss then go and so on. also i stopped at a shop on the way home and when i came back out and started the car and continued home it didnt happen. also id didnt seem to happen in 4th or 5th under load tho that may be coincidence as i hadnt that long to drive the car to try and figure out exactly when/where/ it was happening. it didnt happen under partial throttle and under load just from about half throttle to full throttle and only up to about 3k RPMs.

now.. i replaced the plugs like 2-3 days ago, distributor cap and rotor arm replaced the other day, plug leads were replaced 3k miles ago so im sort of running out of ideas.. it seems to be alot better and this is the first time in the last 2 days since fitting the new cap and rotor arm that i have had any bother whatsoever.



all i can think of as possible causes are:

fouled plugs (old plugs had black deposits on them, assuming blocked breather played a part)

breather blocked or partially blocked (replaced whole breather system but didnt try to clean out the passages in the block where the oil trap connects up to)

loose HT lead connection on plug lead/plug/dizzy/coil (will check them)

vacuum leak (shouldnt be but will double check)

faulty ignition coil (my mechanic buddy seems to think if the coil was faulty it wouldnt run at all, also i have no problems starting the car)


i wont be able to get it plugged into a diagnostics machine till friday so till then im going to keep a close eye on everything and see if it happens again and also check the condition of the plugs (and possibly clean them even tho they are 3 days old), check vac lines and breather lines (used silicone hose to join oil trap to inlet elbow and end of manifold vacuum port to inlet elbow, maybe melted or cracked?), check connections of HT leads to plugs/dizzy/coil.

anyone have any other ideas?

[testrider]

I'm sorry to hear you've gone through all that and still not fixed it. When my 850 started with those symptoms I bought new (genuine) leads and that fixed it straight away. Maybe you got some duff leads when you bought them?

I had a 740 years ago and I changed the plugs, leads, cap and rotor and I got about 3 miles before I had to call out the breakdown truck. It had a terrible missfire and no power so the mechanic refitted the old leads and it worked perfectly.

Got to be worth a try.

[DmcL]

the leads are OEM spec bougicord's, doubting they would be the cause as they were replaced 3k miles ago and havent had any issues after fitting them (prior to the hesitating/missing) also the old leads went in the bin a good while ago so couldnt try them.

[stuartvolvo]

i would definatley say its not your ignition coil, as it would be more than the very light miss which you are getting. Basically your on the right lines with everything you have mentioned. Dirty injectors, fuel filter could also cause slight miss, if not running right. I would double check everything that you have already mentioned and then take it from there.

[DmcL]

its definately not fuel filter as that was replaced under 1k miles ago. im thinking its gotta be something stupid.

most likely causes im thinking are vacuum leak, loose connection or fouled plugs possibly due to partially bunged up breather passages in the engine block (any ideas how to clean these passages? was thinking bit of hose and try to blow engine flush thru the passages ala reverse beer bong style?)

[stuartvolvo]

if there was a vacume leak, i would have thought that you would have noticed it being down in power as well,.
sounds like you know what you doing though.
I would just double check everything, and either give plugs a good clean, or replace, as i have seen faulty plugs before, on quite a few ocasions.
regards
stuart

[DmcL]

it was also doing it with the old plugs (NGK R twin tips with a mere 3k miles on them) before the plugs were changed.

just been noseying the net and stumbled onto this.. doubt it would be the cause but its worth checking for the sake of 1 minute and a cable tie. thinking it could be a remote possibility as i had all that stuff unplugged and moved around, etc.. i havent had an intermittant on/off check light but then my check light has been on due to an "air pump relay intermittant fault" so if this was the case the light is already on so wouldnt go on/off as mentioned below. suppose its just 1 more thing i can eliminate from the list of possible causes.

Some info for the older 850 owners. Recently got an intermittent jerking sensation along with a flashing check engine light. A code 214 referencing the RPM or Crank sensor led me to changing this. When the problem persisted I went to Alldata online, keyed in the code 214 and my car info and what do you know. All I needed was a zip tie to move the crank sensor wire away from the distributor cap. A TSB was referenced from back in 1995.

also another remote possibility i stumbled onto (seems more feasible than the above as my car is late 97, registered 98 on log book).

There is an ECU software patch for motronic 4.4, but it's not for a bad idle. It's for a hesitation, usually when up to speed...In my car, it happened around 70~80mph on the interstate...just feels like a slight miss.

The engine hesitation is found in a handful of '98 S/V70's and C70 ECU's only with certain chasis numbers made at certain factories. The chassis numbers seem to correspond to early '98 Volvo's. By mid-year '98, the ECU issue was fixed so there is no ECU patch for any other Motronic 4.4 ECU's after this time period.

can anyone tell me does the 2.5 10v use the motronic 4.4?

[Chris_Rogers]

No it has a Fenix system.

However, I've never seen a UK 10V car with an air pump, only seen it on the 2.5 20V Non tubro with Motronic 4.4.

[DmcL]

in allthe work that i have done to the car (head off, changed coolant hoses, changed oil and filter, etc..) i havent seen anything remotely resembling the air pump that i have seem pics of on the net. where exactly is the pump located?

the code i was getting was 4-4-2 "air pump relay int. fault" tho the code reader that was used didnt read the codes straight away when volvo 850, year 1997 and engine was selected.. ended up having to try other volvo ones on the diagnostics computer and if memmory serves we only managed to get the code up either under a different model or under the search/automatic selection type mode on the computer.

this is the reader we used.. also will be this reader we try to pull the codes with this friday. managed to erase the 4-4-2 but the light came back on about 2 weeks later, assuming its for the same code.

its a Launch X-431

dont suppose anyone has had any experience with one of these? seems like it should be top notch stuff going by the price tag (a mere £2000 lol)

[NI_Volvo_Nut]

I have a cheap fleebay special code reader, does the job on the s70 though, if you're about tomorrow i'll plug it on and show you the air pump, its beside the throttle body, or close to it, drop me a text, i lost your number, i'll send you a pic of one.