• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

B21FT/K-Jet: Miss at cruising speed, starts hard warm

E

eyeroll63

New member
Joined
Oct 11, 2017
Location
Portland, Oregon
I've been chasing a stubborn issue with the B21FT in my '82 245 Turbo wagon over the past few months. Stone cold, it will start quickly and it runs beautifully, no missing, no stalling. But, once warmed up, you can feel the engine misfiring at steady speeds, especially between 2,000 and 2,500 RPM. If you accelerate and build up boost it will smooth out, but misfire returns again once at cruising speed. You can feel the car jiggle in the seat, not severe, but persistent and really, really annoying! It has been slowly getting worse over time.

I've done a lot of repairs over the past several months while restoring my car. I've also run a lot of tests to diagnose this particular problem, but haven't found the culprit.

Here's a laundry list of repairs, tests and new parts so far:

New fuel injectors, checked for correct spray pattern and output on car, feed lines appear to flow well.
Professionally rebuilt K-Jet fuel distributor
Rebuilt cylinder head and turbo
In-dash A/F ratio meter
New oxygen sensor for Lambda system
New factory catalytic converter
New main fuel pump, upgraded in-tank pump, new fuel filter

K-Jet system and control pressure readings have been textbook so far. Mixture set with new digital dwell meter. Frequency valve checked, along with wiring. Warm-up regulator has been checked and re-checked, control pressure changes verified, swapped with other units, no change.

Dry compression is a bit lower than spec, but all cylinders are within 5 PSI of each other.

Once the engine is warm, readings on the A/FM meter show the Lambda system consistently pulling the air/fuel ratio back to 14.7 to 1 without difficulty.

I've been all over and back with vacuum hoses and connections, replaced many, even found a cracked intake manifold gasket, replaced, no change. Throttle body was cleaned, checked for worn bushings, leaks around sealing surfaces. Throttle valve readjusted.

The ignition system has been treated to new distributor cap & rotor, new wires, new plugs. Ballast resistor was checked, three different ignition control units swapped in, wiring and voltage checked, distributor impulse sender and air gaps checked.

The constant idle system was checked several times. I disabled it for a road test at one point, engine still missed.

I'm running out of ideas here!

Now I'm looking over the ignition system again, currently eyeballing the distributor. It seems to check out to spec electrically, but could there be a problem with the centrifugal advance? Worn bushings?

Really out on the edge here... what about the fuel accumulator? It holds residual pressure as it should, but could it be affecting control pressure to the fuel distributor while driving?

I'm going to re-check system and control pressures on the K-Jet with my gauge set to see if there's something I missed.

In the meantime, I am open to suggestions to help diagnose.

Thank you for reading this far! Much appreciated.

VAH
Portland, OR
 
Last edited:
Sounds like a lean mixture when warmed up but off boost. Are you using the special kjet fuel pressure gauge and following the mixture adjustment procedures in the Volvo green shop manuals?
 
johfraser said:
Sounds like a lean mixture when warmed up but off boost. Are you using the special kjet fuel pressure gauge and following the mixture adjustment procedures in the Volvo green shop manuals?
Click to expand...
I'm using a newer fuel pressure kit set up for k-jet, with the proper bypass valve to check system and control pressures. I'm using the digital dwell reading to set mixture. I have both a set of green Volvo manuals and the blue Bentley, usually stick with the procedure in the Bentley to set mixuture as I don't have a CO meter as shown in the green manual.

My AF/R reads 14.7 pretty consistently while driving once warmed up, I've been keeping an eye on it, but haven't seen much change unless under boost and the mixture is richened by the pressure valve. I have wondered a few times if it would be happier with a richer mixture. I haven't really seen any further info on what this setup (early K-Jet turbo, no intercooler) likes to see.
 
Jack said:
What cap/rotor spark plus and wires are you running
Click to expand...
PS: I ran a set of new rubber-boot Bougicords for maybe 8 months.
Total crap. Avoid at all cost.
One boot tore apart on removal, another had disintegrated inside, insulation had turned to rock and terminal had melted.
Unbelievably awful. Replaced with what I listed above.
 
bobxyz said:
You didn't mention anything about setting & checking the ignition timing?
See TP30432 from ozvolvo.org/archive/ for ignition curves and test procedures.
Click to expand...
I checked static timing after reassembling, looked good, didn't see a change from previous. But, I should check it again, which means putting my timing belt cover back on so I have accurate marks to work from, left off so that I could play with my adjustable cam gear.
I haven't gotten into ignition curves up to now, sounds like this might be a good time to dive into that.

Thanks for the link!
 
eyeroll63 said:
I checked static timing after reassembling, looked good, didn't see a change from previous. But, I should check it again, which means putting my timing belt cover back on so I have accurate marks to work from, left off so that I could play with my adjustable cam gear.
I haven't gotten into ignition curves up to now, sounds like this might be a good time to dive into that.

Thanks for the link!
Click to expand...
Oooo adjustable timing gear. Did you shave the head and that is why you have the gear? Sounds very much like a timing issue, and I wonder if the adjustment you made to cam timing is either off because of a lack of indicator marks and/or mismatch with distributor timing.
 
I got my adjustable cam timing gear along with my IPD turbo cam, but have yet to play with adjustment, it's still in the stock position. Checked torque on the adjustment bolts on install. I've been focusing on getting the motor back to Stage 0 before I start making any changes!

Only enough shaved off of the head to clean it up, just a few thou. Not going for max performance here, the emphasis is on making it a nice street car. Get this: No intercooler. It's still on the original factory non-intercooler setup. I may try one someday, but it's way down on the list. And, frankly, I kind of like it just the way it is! No one else has one like this these days.

Updates:

I swapped out my fuel accumulator a few days ago with a unit from my parts stash. Not exactly the same accumulator, but close enough to install for diagnostics. That took care of the starting problems, so I ordered a new, correct accumulator (matching part number), should be here later this week. One problem down.

I then modified my timing belt cover, cutting it in half just below the water pump, keeping the lower section with timing marks. Sure enough, the timing was off, read about 7 or 8 degrees BTDC when it should have been 12 BTDC. The last time I'd checked it, it was to spec, and I haven't disturbed the distributor position since, so a bit surprised it was that far off. Once I adjusted and reset idle speed, I took it for test drive.

Result: The intermittent miss is still there, but much, much less pronounced. Almost but not quite cured.

Though I've been over vacuum connections at least a dozen times, my next move is to attach a smoke tester to look for leaks. My A/FM reads just a touch lean now after tuning it a bit on the rich side, leading me to think there's some unmetered air getting in and causing the miss at cruising speed that I've been chasing. But, I'm much closer to narrowing it down now.
 
Last edited:
Interesting.
Perhaps the chap who runs Grey Goose Restorations might have some customers with a similar setup and similar experience with that miss while cruising at those engine speeds.
 
Update: Still chasing a misfire at steady speeds. Runs great under boost, but when cruising, I get an intermittent miss.
(there's all kinds of details above on previous work)

My new fuel accumulator arrived, installed a few weeks ago. That took care of most of the starting problems, so one win there.

While checking the charge air pressure switch, I didn't like the look of some of the wiring, so I grafted in some new wire and connectors. This switch grounds CI ECU terminal 7 for boost enrichment, the thermal cutout switch grounds the same terminal for cold start enrichment.

When the misfire occurs, my A/FR meter usually reads 14.8 to 1, drifting only occasionally down to 14.7, which seems a bit on the lean side.

I tried resetting the mixture at the fuel distributor toward rich, lowering the dwell reading down to just under 40 degrees. But, as I suspected, the Lambda system compensated, pulled the mixture back to lean, and I was still getting 14.8 A/F.

I decided to try an experiment: wiring up an in-dash switch to ground terminal 7 of the ECU so I could temporarily activate the enrichment circuit at steady speed, off-boost. This pulls the A/F ratio down in a hurry, all the way to 10 to 1. Result: once A/F falls into low 14 range, the misfire ceases.

I understand that a misfire can cause the ECU to detect unburned fuel at the oxygen sensor and lean out the mixture. But, I've been over the ignition system, vacuum lines, fuel injection, fuel pressures, warm-up regulator, continuity checks on wiring... everything seems to be to spec. And, again, it runs beautifully under boost, pulls just fine, no miss and no miss after starting when it's stone cold, I suspect both are the result of enrichment via the thermal cutout switch. It's just at steady speed, after it's warmed up.

Aaaaaagh! What could I be missing?
 
Last edited:
Maybe try some Marvel Mystery Oil/Techron fuel additives and/or google "Italian tuneup" ?

B21ft K-jet Questions: Frequency Valve and Related Systems

I've been doing tons of research and trying my best to figure this out before posting here, but I just can't get the conclusive answers I need as a K-jet n00b. Facts: - 1983 245T intercooled running quite rich most of the time - CPR taken apart and cleaned - recently new injectors - all new...
turbobricks.com turbobricks.com
 
You must log in or register to reply here.
Back
Top