Fuel Management

I have encountered a strange problem with the fuel management on my 6.9.K Jetronic...something weird is going on.

All aspects of the fuel delivery system have been quadrupled checked and everything is in spec.

I have a recent overhauled fuel distributor: the injectors have only done 5K but recently cleaned and checked/all good.

BACKGROUND Following a recent top overhaul, the whole injection system was reinstalled, checked, re checked etc and bled etc . All was well but no top end power. Then all of a sudden, an intermittent miss presented on 2 cyls under light load and beyond.

This was finally ratted down to be air in the fuel lines. This was bled out only to repeat itself a day later?. A different bleeding technique remedied the miss AND restored the lost top end power.[:0]
[sure as eggs break, I'm not complaining about that!!! [:D]]

So what's going on? How is the air getting in there? If the system has had air trapped in it for weeks, how perculiar that it manifests in low top end power, but occasionally morphs in to a couple of injectors firing blanks?

The only explanation I can think of is that altho the whole system should be self bleeding over a few cycles, , maybe some air got into it when it was all dismantled for the overhaul. When re-assembled, despite manually bleeding it a couple of times, a bubble of air still got trapped some where and has prevented the fuel from being fully delivered. A bit like "The Bends".. the air bubble was trapped in part of the distributor but has been moving around. The question is, have I fully expunged it, or is there some weirdo air leak?

There are many questions but the obvious one is how can a system with air in it, present perfect fuel pressure values under load?

Any insights would be appreciated.

After several months since posting this page, I was still plagued by poor top end performance.

So there was no choice. Yep!..The full blown in situ test of the fuel distributor and its output.

This project took 2 half days. Not for the feint hearted. It's not hard, just a slog.

I obtained some fuel output values from a learned specialist in Sydney altho as it turned out, it wasn't about how much was being delivered but rather the variation between ports.

To cut 2 days of pain down to a few lines, I was suspicious of the recently recond fuel distrib. It actually turned out to be performing well. That is the output volume was pretty consistent across the 8 ports at half and full volume.

Yet, when the newish and recently cleaned up injectors were fitted, the variation was ridiculous. Quite astounding considering they were only 8000kms old and were cleaned only 4000 kms ago.
It's a wonder the engine ran at all.

So after much cleaning and testing, i got the injectors somewhat improved but by no means consistent.

Remarkably, the top end performance is back with vigour.
When you consider at least 3 of the injectors were delivering only 20 cc instead of 150, I realise now what i thought was weak detonation under load was detonation. lean mixture big time.!

Whilst vastly improved, only 2 of the injectors are within cracking pressure: a pretty motley lot really so it's a new set.

I reckon she is going to fly with a new set: all cracking at 45 psi and delivering a 150cc per minute. No wonder only the very wealthy could run these at autobahn speeds.1.2 litres per minute at 200 kph!

I would like to advance a couple of conclusions.

One collateral issue relates to idle mixture and smooth running.
After the injectors were cleaned, not only was the top end enhanced but the idle went cranky. This is a consequence of hily variable flow rates at low air speed compensated by setting the mixture control too rich. This observation is relevant to those seeking a superior idle. That is, you will never get it unless all injectors are cracking at the right pressure and the volume delivery is within 10% of each other. The spray pattern must be spot on and no dribbles.

The other matter, touched upon earlier, [which is serious,] goes to the fact these monsters will pull like crazy even with rotten injectors.It's only when you lean on them hard that the upper rev range looses its lustre. Extended lean mixture is not a good thing:equally, you don't want one or two that runs too rich.

There is an old post on this Board where people speculate why some 6.9's "fly" more than others."Smoke the tyres" talk.

It's simple: Even compressions...even and full fuel flow...correct spark. The most overlooked I suspect is the fuel flow because it's such a process to double check.

Anyway, it seems to me there is merit in doing a flow test every so often to ensure you are getting peak performance. Any K jet will love you for it.[;)]

I will update this post when I get the $$ and energy to replace the injectors.[:(]

ps in case u r wondering how the injectors came to be so gummed up so quickly, I do not have the answer. I do run ATF in the fuel from time to time.usually 95 ulp from hi volume servos.all filters are in place.......beats me.[V]

Nice one Randy[:D] you continue to blaze the trail!

Daniel
1977 450 SEL 6.9
Vancouver, BC

Daniel, thanks for your enthusiasm but this procedure has been around awhile.I am merely re-clearing an old trail that has become overgrown with the passage of time and ebbing "corporate memory".![:D]

This would be a fun project on a Ferrari Testarossa, with 2 FD's and 12 injectors?!

A point to be made here, is that whilst it is great to have your FD bench flow tested- which is about $150 plus shipping both ways-
the process I outlined tested both the FD and the injectors in situ and revealed the treacherous injectors were/are the culprit.

I have busted open the piggy bank and new injectors and seals are on their way!!. We shall bench test the injectors before fitting. I'm jack of getting dodgey new bits: all 8 have to be picture perfect coz if one is shonky, it's half a day's work to re-diagnose. I'd bet London to a brick, most new ones are fitted up straight from the box without question....no longer good enuff.

My next party trick is to create the secret ambrosia to purge and clean old injectors. I tried everything on mine and altho vastly improved, it's like being "a little bit pregnant". Near enuff does not cut it.[^]
This exercise can canter along in the background after I fit the new ones: to be reported on in due course.

OK.got the new injectors and seals and bench tested the injectors. 2 were stuck. it is unclear to me how they would have become unstuck if fitted up because it took a bit of over- pressure to get them to play ball.

Anyway, all now picture perfect...cracking at 40-45 psi.......beeeeutiful !!

Fitted them up, did some adjustments and took it for a burn. Indeed, top end wind out enhanced even further and it now sounds and feels like it might have 30 years ago. The temptation to let it over rev needs some discipline.

Whilst the idle is marginally better, it is still not as good as it should be. This pest of an issue has defied everything...there is only 1 shot left in the locker from the fuel management aspect and that is to re test. groan.[V] the fuel flow at idle values to determine the accuracy of low volume flow thru the fuel distrib.

Hi Randy

very interesting reading.

I might have to visit you and feel the top end.[:D]

Getting closer to getting the blue girl back on the road

cheers

Andreas

1)Australian deliered 6.9
2)3 x Euro delivered 6.9
Help M100 car yard

Part 3..altho I've been here 20 times already.[xx(] .

Not happy about this and it illustrates a too often recurring problem- either cruddy aftermarket new stuff OR dodgey recons.

Here goes.
I got around to testing the idle fuel flow values for the recently recon'd fuel distrib. {it's done <400 kms }

Having sorted out top end issues which were related to gummed up [yet recently cleaned and tested injectors,] my attention now returned to the vexing idle issue.

Imagine my surprise when the testing revealed fuel flow 100% more than they should be from 2 ports. [:0]
Over the past 18 mths I have done everything to tackle the idle, based on the premise that the fuel distrib was 100%, therefore, the idle issue is something else.

Well, the something else is about a 100 possibilities and having forensically crushed each them to death, it left, purely by default, the FD. It has taken dozens of manhours and 100's of dollars diagnosing this.

So here we go again. Another recon unit from a local guy who seems to know his stuff BUT as outlined earlier about testing injectors before fitting them ,I will be testing the flow pattern of the FD each step of the way.


I wouldn't trust a spark plug to come out of the box and work these days.

So I shall report in, once the new FD is fitted up. I don't hold any hope for any recourse on the dodgey one I'm replacing, altho I sent a scathing Em to the supplier.

These cars are complex and difficult enuff with out some Jive Turkey supplying treacherous componentry.(!]

Morale of the story folks: quality control has been outsourced...to YOU!

It can be a major pain the arse all right. Especially when the FD is alledgedly a good one . I have often ended up making my own from a box of dead ones .Although the worst had to be a Porker i did . The Fuel injection was gummed completely after sitting for 10 years and the 928 uses a very long injector. In the US they dump those and fit ....Mercedes ones. So I did that but the bugger wouldn't run. I swapped on a Mercedes 380 FD and bingo ..it still has the dodgy pats but runs great , probably better than it should .Then there is the bloke near here who claims to be able to rebuild the distributors. Someone,who i wont name, sent a car to him which was being difficult to start ( I said "dirty fuel filter" ) the car was picked up after the 5 figure bill was paid .And promptly broke down outside the work shop in huge cloud of black smoke. It still isn't repaired. It was the second one I'd heard of that died there, the other was hydrauliced with a load of excess fuel and bent a rod.... [xx(]

quote:

---

12-14-2004, 11:49 PM #8
Tom Hanson
MBCA Member

What the heck, try to stuff a MB 6.9 liter V8 in it. What a machine that would be..
__________________
Tom Hanson
Orange County Section

---

Yes Ron, I tried to rebuild one...actually..2 and they were real dogs...would not seal? Can't figure out why?.

They were face honed on the thigh of a cuban cigar roller and swathed in banana leaf extract until they shone like chrome.
After 2 kits/missing shims and springs boing-boing all over the place, and hours wasted fiddling round, I thought I'll get a recon. And that's when the trouble started as they say........
I hope the place in Qld where i am getting my FD done is not umm err the one you are referring to!!. [:0]

Arrrggghhh!
. This is a guy who I have known for 30 years .His father inlaw was a Genius who figured out years ago how to rebuild the fuel distributors and even made parts that bosch wouldn't supply .
[urlhttp://bowerpower.com.au/[/url]

quote:

---

12-14-2004, 11:49 PM #8
Tom Hanson
MBCA Member

What the heck, try to stuff a MB 6.9 liter V8 in it. What a machine that would be..
__________________
Tom Hanson
Orange County Section

---

UPDATE

To quote Richard 111 " A fuel distributor..a fuel distributor: my Kingdom for a working fuel distributor"!!

Yep..after waiting 3 weeks to get an exchange FD, ,the replacement sure looked the part but thence it couldn't talk the talk let alone walk the walk.
It fritzed within 10 seconds of installation. ! [:0]

Changing one of these over should be once in a lifetime. This was my 4th. 2 second hand runners which were dodgey. One ex US recon.:good-ish but no cigar...and now this one which is worse than one I rebuilt unsuccessfully myself?!

I don't car how slick you might be as doing this changeover. I've had to do too many and it's a sod of a job: 3 hours as you have to pick your way carefully.........and double check everything.

And so I tried to check the free flow output before hitching everything up.......it didn't seem right...damn it!.it wasn't right...far too much flow. WT*?

Ah well, soldier on. let's see what happens. Ha.! it ran like a sick dog.....shaking/missing/popping/rich /lean...out of control.

So what happened here? I reckon it can be traced to a faulty press reg valve.......it allowed instant overpressure [ 7 bar+ ] and that in turn goosed the flim flam shims in head itself. Far too many shims on the PRV IMHO..I swapped it over with my old one & it was instantly 5.6 bar. Too late she cried..[xx(].......the innards were ruined.

So. Off it comes again....back it goes...we wait..back it comes....we refit: and here's the best quality control check you can do as the mug at the end of the food chain...cross ya fingers!!!

Sounds professional doesn't it??