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Odd occasional hesitation under acceleration/boost

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Old Jan 17th, 2020, 02:17   #31
AllHailKingVolvo
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Originally Posted by Laird Scooby View Post
Yes, just one pimp in the tank Phil, no pre-pump/in-tank lift pump or whatever you care to call it. Changed in 1995 i believe to just the one pump.

If the new filter has improved throttle response and power that much, arguably once the injector cleaner has done its stuff, it should be ok but it's worrying that the fuel filter was as bad as you describe.

With that in mind, i'd suggest also checking the condition and tension of the timing belt. The fuel filter should be changed every 2 years/24k miles (although Volvo often seem to work on 10k/year mileage) so it should only have been 5k miles old if the current mileage is 77k. It seems as though it hasn't been done since 48k miles (if then!) from your description so implies other maintenance tasks have been skipped or not done properly.

My old 740GLE had allegedly had the head gasket done with a new timing belt under a year before i got it. I noticed every so often, particularly when it was working hard, the performance would drop off. Knowing everything was as it should be, the only thing i hadn't checked was the timing belt and when i did, it was flopping about like a loose twig on a tree. That prompted me to also check the tightness of the cylinder head bolts. Glad i did as none were done up properly, it's a wonder the thing ran at all!

Ordered a new timing belt and didn't use the car until it arrived and was fitted, sorting the head bolts in between whiles. Transformed the car, performance, economy and response much improved and no more flat spots/lack-lustre performance when it was working.
That's a very good point re the timing belt. Service history states it was changed 14k miles ago, but the car has done so few miles that that was nine years ago! Definitely time for a fresh belt and tensioner kit methinks.
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Old Jan 17th, 2020, 10:13   #32
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It appears that we have something of a breakthrough! I took the 940 on the school run this morning (mainly to give the injector cleaner a chance to work through the system), and with the fuel filter change and Forte in the tank, it feels much improved.

There is still a slight unevenness under heavy load but nothing out of the ordinary compared to my other Redblocks. The car has plenty of power on tap and throttle response is much improved.

I think with the new CTS, CPS and possibly a new Lambda sensor at some point soon, it will be as close to perfect as its going to get.

Knowing the relative shortcomings of the ignition system, it makes me tempted at some point in the future to try out a wasted spark conversion. I do quite a few miles, and the proposed MPG gains would pay for the system within a year, plus smoother running is always appealing!
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Old Jan 17th, 2020, 12:05   #33
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Give it a few days use for the injector cleaner to do its work Phil then pull fuse #1 for a few minutes to clear any fuel trims learned from the clogged injectors time.

Also when you replace the CTS pull it after as well, likewise if/when you change the Lambda.

Great you seem to have some progress on it!
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Old Jan 18th, 2020, 16:43   #34
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Originally Posted by Laird Scooby View Post
Give it a few days use for the injector cleaner to do its work Phil then pull fuse #1 for a few minutes to clear any fuel trims learned from the clogged injectors time.

Also when you replace the CTS pull it after as well, likewise if/when you change the Lambda.

Great you seem to have some progress on it!
Will do! I'll be fitting the CPS tomorrow and changing the gearbox oil, CTS should arrive on Monday so will fit that and chew through the injector cleaner, and hopefully this stage of the recommissioning will be complete...looking forward to enjoying the car and not chasing niggles, although to be honest that is of course half of the fun with owning these lovely old machines!
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Old Jan 20th, 2020, 08:25   #35
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Hmm...took the car on a good run yesterday (a 2hr round trip) to work and back, and whilst it generally drove well on the way there, the misfire/flat spot had returned noticeably on the way home. However, it seemed to come and go periodically which seemed odd.

Yesterday on the car I changed the CPS and cleaned all earth strap connections all round. It ran fine when tested afterwards and generally well on the journey to my gig, it was the trip home (temperature much colder and more damp than the journey there).

I'm wondering if initially, the more free-flowing fuel filter increased fuel pressure which richened the mixture, but subsequently, if the CTS is off, the ECU has leaned out the fuel trims again as it's "learned" the new fuel flow rate etc.

I'm also suspicious of the Bosch HT leads I fitted, the boots on the king lead and cap end aren't as long and nicely fitted as the ones that came off, which themselves were cheap leads...so I bit the bullet and ordered a set of Bougicord units. I've read some negative comments about Bosch leads so figured going OEM would probably be best.

The hunt goes on...I'm determined to get to the bottom of this!
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Old Jan 20th, 2020, 09:53   #36
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See if you can work out a pattern as to when the misfire/flat-spot appears Phil and once you have, see if you can recreate it by simulating the circumstances around it, that will help enormously towards hopefully finding the cause.
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Old Jan 20th, 2020, 14:21   #37
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See if you can work out a pattern as to when the misfire/flat-spot appears Phil and once you have, see if you can recreate it by simulating the circumstances around it, that will help enormously towards hopefully finding the cause.
Thanks Dave, I will do! It seems pretty arbitrary at the moment but will see what I can deduce. It ran fairly well this morning but I noticed that it was producing a bit of grey smoke on startup and for a few mins whilst I was clearing the ice from the screen, rather than the crisp white smoke I am used to seeing from Redblocks on a cold start.

It wasn't blue in any way, so it isn't burning oil, but it looked to me like it was the symptoms of running a bit rich...quite the opposite problem to what the plugs seemed to indicate at the last change!

I'm going to keep an eye on it, new CTS arrives tomorrow and Bougicord leads the day following, so will see how they fare.
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