l-jet Revealed, Now how does this really work? Options

[jesiv]

Porsche produced the 914 1.8 for two years (74-75) with two different FI. Every time I talk to someone regarding the l-let, they are quick to point out two things. First, the l-jet is the best FI for the 914 (also the basis for all future FI, used on the 912 2.0L, AFC vs MPC, etc) and it hates vacuum leaks! Which is great? But does not address the fundamental lack of information for Porsche 1974 and 1975 914 regarding their l-jetronic fuel injection system.

I have searched high and low for information regarding l-jet. It has been hell trying to get basic information that in the d-jet world widely documented considered simple and widely available. I dedicate this thread to the “Full Disclosure” -- l-jet revealed!

I am hopeful that we can put together a thread that can actually help 1.8 owners

The glove has been thrown…

Regards,

James

[Sleepin]

I will watch this thread with interest!

[r_towle]

If you look at the top of the page, in the 914info page.
Click on other 914 sites.

guess what you find.

Three awesome sites.

This one is all for L-jet

http://manuals.type4.org/ljet/

Also, get the Bosch FI book.
It is really a great manual and it explains Bosch FI very well.

Rich

[jesiv]

Yes, I agree, but if you really look at it.. It is not specific to the 914. That is what I am hoping we can address. Is the very specific application of the l-jet to the 74-75 914. Like the Porsche 914 l-jet version is an open-loop not a closed loop...

The goal is to keep the thread specific to the 914 not other implementations of l-jet

[JeffBowlsby]

Hey James, be sure to review the L-Jet documents I have on my website if you have not seen them:

http://members.rennlist.com/914_collectibles/RareDocs.htm

I would not go so far as to say either L-Jet or D-Jet was 'better', I think they are both good and have their own strengths and weaknesses. L-Jet was a more developed system and improved over D-Jet in several ways, but a D-jet was a good system.

L-Jet characteristics that come to my mind:

*Fewer components which were to make a simpler system, but those components were more complex.

*The hardware design was significantly better over D-Jet and upgraded variations are still used in modern cars = spring loaded wire terminals, no reliance on a fragile rubber boot for protecting the connector wires and connections, but the harnesses are 2x as complicated as D-Jet harnesses.

*The double relay component replaces some of the function of the standard 914 relay board.

[ConeDodger]

If I recall correctly, L-Jet uses a air flow meter that is really measuring he position of the flapper door in the AFM itself on a potentiometer. It also uses a throttle position switch much like D-Jet. It measures cylinder head temperature. The injectors fire for a percentage of time and that percentage goes up as the need for fuel as measured by the flapper door, the throttle, and the head temp dictates. The L-Jet on 280Z cars has dropping resisters to bring the voltage down to what is needed to fire the injectors. I am not sure if the VW Porsche L-Jet does.

I don't believe one is superior to the other. With the possible exception of the fact that we don't seem to be able to get berylium to rebuild the MPS on the D-Jet. The L-Jet seems to be easier to get rebuilt...

I hate them both. More patient men will disagree.

[Rusty]

Once upon a time, Alfred used to maintain a bunch of l-jet information.

His site is gone, but can still be accessed through archive.org.

Use this link:

http://web.archive.org/web/20050913031343/...nk/earlyefi.htm

[Mikey914]

I'm impressed this is an excellent repository of Ljet information. I happen to be working on an L jet right now. Unfortunately I have several ECUs and I've mixed them up so, can anyone tell me the numbers for L jet ECUs?

ConeDodger - you are correct on the operation of the "barn door" air flow measuring. This is the same as the 944 and 944 turbos. One substantial upgrade for the turbo is the elimination of this unit with a mass airflow sensor. It's supposed to be about 8% gain in HP.

[type47]

Unfortunately I have several ECUs and I've mixed them up so, can anyone tell me the numbers for L jet ECUs?

i looked in the PET and found illustration 901-05 listing 2 ECU's:

for engine numbers up to EC0 037 551 the ECU is 022.906.021G
for engine numbers from EC0 037 552 the ECU is 473.906.021

my guess is the 022. is 74 and the 473. is 75

[McMark]

If you're technically minded, this is the book to get. It covers a ton of technical information about Motronic, L-Jetronic, LH-Jetronic, LH-Motronic, D-Jetronic, K-Jetronic, KE-Jetronic, and KE-Motronic.

Great stuff.

(IMG:http://ecx.images-amazon.com/images/I/51A96M540ML._BO2,204,203,200_PIsitb-dp-500-arrow,TopRight,45,-64_OU01_AA240_SH20_.jpg)

[ConeDodger]

I'm impressed this is an excellent repository of Ljet information. I happen to be working on an L jet right now. Unfortunately I have several ECUs and I've mixed them up so, can anyone tell me the numbers for L jet ECUs?

ConeDodger - you are correct on the operation of the "barn door" air flow measuring. This is the same as the 944 and 944 turbos. One substantial upgrade for the turbo is the elimination of this unit with a mass airflow sensor. It's supposed to be about 8% gain in HP.

I hear the same modification is available for my '84 3.2 911. But the fact that I have to do smog here in California and the fact that I have been threatened with great bodily harm or death if I modify anything on this 55K mile car has kept me from doing it. I hear it smooths out the idle and really wakes things up!

[Mikey914]

Challe was nice enough to ponit me toward this link
http://www.pelicanparts.com/914/technical_...4_18FI_diag.htm

[Brando]

Switching over to a MAF setup on Motronic (for 80s carerras and 944s) and MAF for AFC on the 914 are very different. Even though the systems are VERY similar, they are just that - similar.

I found a place that could make me a MAF sensor and the interpreter box - for only $800 and the cost of them scrapping an ECU and AFS. It would work great on 914s because they're not closed-loop controlled by an O2 system, and most have no smog limitations throughout the country.

[Katmanken]

L-jet works good and it evolved into most of the systems used today. Manifold pressure wasn"t where it was at and Bosch evolved. The flapper box works good but has lag. Backfiring through the intake damages the flapper box. Flappers were later replaced by hotwires to measure airflow and they eliminated lag and backfirng issues.

I have a later version called Digifant on my 1986 wasserboxer Vanagon and it is bulletproof. Looks just like the one on my teener.

Ken

[r_towle]

Going back to why L-jet is open and JH jet is a closed loop.

I think its an emmission issue given the timing.
EFI was forced evolution by the EPA...take a look at carbs from the 70's through the 80's
By the end, the carbs had 20 vacuum lines running off of them and were impossible to diagnose, it was a rats nest of sensors etc.

L jet was simple, and it cleaned up the mess of lines, and lowered the emissions.
the evolution to LHjet, with a lambda sensor was a natural evolution to keep up with the progressing laws of the time.
I think that LH jet was used by the japanese cars for years.

Rich

[type11969]

Wouldn't the l-jet on the t4 VW buses be basically the same as the 914 l-jet? May want to check the VW forums for more information, thesamba, shoptalkforums, etc.

I'm reading that Bosch book right now to try to understand the l-jet in my Bus better.

What triggers the injection event in ljet? I haven't gotten very far in the book yet!

-Chris

[Bleyseng]

Wouldn't the l-jet on the t4 VW buses be basically the same as the 914 l-jet? May want to check the VW forums for more information, thesamba, shoptalkforums, etc.

I'm reading that Bosch book right now to try to understand the l-jet in my Bus better.

What triggers the injection event in ljet? I haven't gotten very far in the book yet!

-Chris

yes, its basically the same, some part numbers are different. I would think you could run the bus Ljet in a 914 but you would have to set up the airfilter.

[type47]

What triggers the injection event in ljet?

the L-jet in my '81 Vanagon is "the same" as the L-jet in my 74 914.

i think the injection is triggered off the distributor. this makes me wonder and ask the question about how the ignition/injector systems are affected if you use a Pertronix, pointless ignition. obviously, it works but if the injectors are triggered off the points closing, and there are no points to close..... aarrrrrrrggggggghhhhh! (IMG:style_emoticons/default/idea.gif) (IMG:style_emoticons/default/confused24.gif) (IMG:style_emoticons/default/wacko.gif)

[type11969]

Ljet at least doesn't have the trigger wheel in the distributor like Djet, I was happy to find this out when I started poking around my new bus because it allows you to run a mallory (at least more easily than djet). That doesn't mean that the injection event isn't still timed off the distributor in some way, but how . . . dunno.

Definitely really digging fi though over the dells in my Beetle. One thing that bugs me about this old fi though is it doesn't have the limp home abilities that carbs do.

[davesprinkle]

Ljet at least doesn't have the trigger wheel in the distributor like Djet, I was happy to find this out when I started poking around my new bus because it allows you to run a mallory (at least more easily than djet). That doesn't mean that the injection event isn't still timed off the distributor in some way, but how . . . dunno.

Definitely really digging fi though over the dells in my Beetle. One thing that bugs me about this old fi though is it doesn't have the limp home abilities that carbs do.

Yes, the L-jet ECU receives engine speed information from the ignition signal. That's why the injection harness has a terminal that connects to the switched side of the coil. Amaze your friends with this bit of ancient injection trivia -- because every coil pulse looks the same as any other coil pulse, the L-jet ECU is UNABLE to track engine position. It only knows the SPEED of the engine. Since the ECU doesn't do, indeed CAN'T do, injection phasing, your mallory will work just fine.

Modern ECUs rely on a crankshaft signal that is modulated in some way (for example, with extra or missing teeth) so that the ECU can deduce crankshaft position as well as speed.