Tardis928

Car: 1984-928S-US
MAF: Bosch 0280.203.015

The car starts and seems to run OK, only when I stick a hose down the MAF to hold the door open.

Here is some background:
About a year ago, after running around a bit, I checked under the hood to find gas all over the top of the motor. Yikes!!! I figured it was time to bite the bullet and replace all the 30-year old, brittle, cracked fuel hoses and lines. While I was at it, I’d decided to go after the vacuum lines too.

So, I replaced the fuel hoses and lines. All the vacuum lines in the motor compartment. Replaced the old injectors with a new set of rebuilt injectors from Five0. New seals where the spider legs meet the body. Basically, if the hose was hard and cracked, it got replaced. I did the radiator and heater hoses a few years ago when I put in a new timing belt.

After putting everything back together and checking to make sure there were no fuel leaks, I tried to start it. It fired but wouldn’t run. After checking a whole bunch of things and not having any luck, I decided to try the old hose-down-the-MAF trick and she fired right up. When I remove the tracheal tube, she dies. When the tube goes back in, she fires up and runs. One really odd thing, though. When she is running, the turn signal/flasher relay is clicking away like a Geiger counter at Fukushima.

I suspect the MAF is not getting any power, or it is not talking to the ECU. I went through Group 24 in the WSM and didn’t come up with anything that gave me a clue as to how to test the power and signal. I saw the article at http://www.the944.com/afm.htm, but dissecting the MAF is a last resort.

Some things, I checked. Fuel rail gas pressure is 30 psi. I have 8” of vacuum at the fuel pressure regulators. 0” of vacuum at the vacuum advance at idle. I don’t know if this is normal vacuum? I had problems finding what the vacuum values for the car should be.

I suspect my problems are electrical and not mechanical. But I’m at a loss on how to proceed. I’ve been through this forum many times but could not find this specific problem (or I failed to use the right mix of search terms).

Any help would be appreciated. Thanks!

Landseer

Terminology-wise, it's actually an AFM, not MAF. AFM has springloaded gate with an mechanical/electrical pickup. If it is not opening, the engine has a rather large vac leak or isn't generating much. Timing could be way off also. Is it possible that a hose at the base of the throttle body might be leaking (did you replace every hose)?

I had the problem on 84 cars, once it was timing (had a gear flipped), another time I had a bad leak.

(MAF is on 32v USA and ROW cars and 84-86 16v ROW cars. Air gets drawn across a trio of heated wires and the electronics make temp-corrected estimates of air mass.)


Please say more about the injector swap. Picture would help. Were you able to get exactly the same injectors original to the 928 LJet cars? I didn't realize FiveO had them, but it makes sense.

Tardis928

Hi Landseer and thanks for the response.

Sorry about the mix up in terminology. I wasn't sure what to call it. When I Googled the Bosch part number I had no less than 6 different names for it. MAF seemed to be the most common.

So the gate on the AFM is spring loaded, not operated by a solenoid or anything like that? Does it need a vacuum on the throttle body side to open during operation? I couldn't find any documentation on how the AFM actually operates. If there is no solenoid, then my issue isn't electrical, instead it is a vacuum issue. I've been looking in the wrong place! In one of the threads I saw how someone rigged up a pressure test of the manifold. I'm thinking of doing something like that to help locate the leak. What is the risk of damaging any parts by doing that?

The injectors from FiveO were remanufactured Bosch and looked like the injectors that were in the car. I've seen FiveO mentioned several times in this forum with no negative comments. I can't say if these are part of their normal product line since I only made the single purchase from them.

I going to check for leaks, thanks again.

Fogey1

Try a few of these links.

GREG'S BOSCH L-JETRONIC FUEL INJECTION IDLE ADJUSTMENT, DIAGNOSTIC AND TUNE UP PAGE
http://www.hiperformancestore.com/Ljetronic.htm

The Air Flow Meter (AFM) - from "The 944 Motronic DME" by FR Wilk ©2001
http://www.the944.com/afm.htm

http://demo.cs.brandeis.edu/postscri...Injection.html

http://en.wikipedia.org/wiki/Jetronic

Limping Home with L-Jet
http://www.ratwell.com/technical/LimpingHome.html

Landseer

If you did anything to the timing belt or distributor during intake refresh, it would be wise to check for timing alignment. If so, let us know, we can tell you what to check.

The intake hoses need to be intact. At the base of the throttle body there are at least one hose (brake vac assist?) that need to be tight. O-ring on interior of throttle body, into-which the AFM mounts, needs to be new. I generally use a little grease on that O-ring to mount the AFM. Important connection.

The grounds need to be clean (pass side cam cover, one of 2 sets of grounds ocluded by an air valve) and the wires attached to the AFM need to not be frayed (not typically a problem).

Tardis928

Hi Landseer,

I didn't anything to the timing belt or distributor during intake refresh. I even modified a socket to get to the bolt under the distributor for the manifold leg. The distributor remained locked down during this entire process. I had to remove one of the spark plug cables from the plug to replace a fuel hose segment.

Again, she fires right up when I let her breathe.

It would be fairly easy for me to pull the hose off the master brake cylinder and check the vacuum there. That would give me the vacuum at the brake assist. Do you have any idea what sort of a reading I should expect?

PS Fogey1 - Thanks for the references.

j.kenzie@sbcglobal.net

The AFM is a swinging door that measures air flow, and sends a signal to the FI brain to match the fuel to the measured air. It kind of sounds like you have a sticky swinging door. You should probably pull the unit and look for binding in the door mechanism, need for lubrication or other misalignment. A vacuum leak would act very much like a stuck swinging door, so check for that too. Rebuilt units and used units are available.
Good luck,
Dave

Tardis928

Hi Dave,

I'll pull it out and double check. It seems to swing fairly freely when I shove a hose past it and retracts when I pull the hose out.

j.kenzie@sbcglobal.net

Hi Greg,
It has to swing freely based on air currents. It could just be sticky, or the vacuum may be too weak to make it swing. It could also have electrical problems.
Dave

Landseer

Vac or timing.

Tardis928

Hi Dave,

Pulled the AFM. The door is easy to push open and it doesn't grab anywhere. According to my trusty kitchen scale, it takes about a pound of force to open it.

Hi Landseer,
Sorry, I'm not sure which part of the thread you are referring to.

j.kenzie@sbcglobal.net

If there is a vacuum leak, there will not be enough air flow to force the door open and send a signal for more fuel. Also, these cars are very sensitive to timing. I moved my distributor a tiny amount to replace the green wire. It barely ran until I put a timing light on it, and set it properly. It could still be a problem inside the AFM. I saw a thread about its inner workings somewhere. I'll see if I can find it.
Good luck,
Dave
BTW I would listen to Landseer. He's a pro. Here's the link to AFM repair:

http://www.the944.com/afm.htm

Landseer

Timing should be ok, but I always like to start there. OK.

If its all in-time, then you have likely got a vac leak. Trace and doublecheck each vac line (hard with intake pipes installed, but it can be done. Do you have a new O-ring where AFM force-fits into throttle body? If not, wrap outer diam of AFM with duct tape until you achieve an interference fit and try to start it. BTDT.

Also on the 84, the thermo/time switch mounted in the water bridge can cause a flood/no start. Smart to replace this too. It will eventually fail and starts below 60F will be very difficult, if possible.

Tardis928

I removed the passenger side fuel rail and 2 of the manifold legs to gain access to the throttle body. When I checked the injectors, I noticed I had mangled one of the tiny injector seals when I did the install.

[Picture of Seal]

Also, in an early post, Landseer asked about the FiveO injectors. Since I had the fuel rail our, I took a picture of the new FiveO remanufactured injector along side one of my old injectors. The FiveO injector had "73303" stamped on it. But it was difficult to read.

[Picture of Injector]

This is my first time trying to post some photos, so I'm not sure how well it is going to work.

Tardis928

Its alive!! Its alive!! Its alive!!

I got her back together and she fired up the way she should. I replaced the mangled injector seal and re-made 2 of the vacuum hoses. Two of the vacuum hoses that I originally replaced were really tight. I cut 2 new hoses longer and they snake around the obstacles better. I also reseated all the hoses I replaced the last time. No obvious "smoking gun", but something I did worked. It must have been a vacuum leak.

My thanks to all that helped me track this down and get her running again.