Hi Folks,
Throw a little problem out there. My Coupe with about 170k miles has been running silky smooth for some time. Couple years back I swapped it to classic registration which limits number of driven days as I wanted to preserve her and her midnight blue Zender kitted glory. About a month back, I took her for a short workout after she had been sitting for awhile.
Ran beautifully, didn't miss a beat.
I stopped at the supermarket on the way home. On coming back to the car and start.....
Bang (like a backfire) and no start. It wants to turn over but just won't do it.
Fuel fine, pumps sound fine, battery fine, starter sounds fine.
Now the perplexing problem.
IF I depress the throttle plate ever so slightly it will start quickly and easily.
Release it and it stops. Depress it too far and it stops.

I have had such a variety of "its the......" suggestions that are far and wide.
Many seem to ignore the fact it starts and idles fine with the slightest depression of the throttle plate. I can't for the life of me imagine how that correlates to a distributor cap and rotor button, which is one of the suggestions. If it wont start due to those things depressing the throttle plate is not going to provide a fix IMO. (I could be wrong of course)

So guys any ideas I can check myself before committing to towing it to a mechanic who will likely experiment until he finds the issues ? Not really cashed up too well right now for expensive repairs and she will just have to sit in the driveway if its a major problem.

Over to you, and thanks in advance.
Zender Merc

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Been fighting a fuel issue myself, about got it resolved. Biggest issue on mine was a HUGE vacuum leak, or multiple ones. I found both idle hoses leaking, at least three injector seals leaking, the injector body to intake boot folded over and leaking, my EHA was leaking and the O2 sensor was shot. After fixing all that and THEN adjusting the mixture, it seems to be vastly improved, but still tweaking.

Yours certailny sounds like an extreme lean run situation. I'd start by inspecting the idle air hoses, checking the EHA for leaks and consider a new O2 sensor. Might also check the idle microswitch. Whatever you have going on, it's fuel/mixture related, from the sound of it.

Big thing is to hit youtube and learn how all the components of the K Jet system work and inter-relate. It's not super mysterious, just totally different from any other modern EFI out there. Last time I dealt with one was in the early 80s on a 72 VW squareback.

Just thought of something else... I had a Nissan 300ZX Twin Turbo maby years ago (fast, but horribly unreliable an miserable thing to work on) that did almost EXACTLY what you describe. You could take a fuel line loose and the pump would shoot am unbroken jet of fuel four feet. Problem turned out to be the pump was producing NO pressure, even though it produced volume and you could hear it running. Check fuel pressure.

I'll give it a look see, but that doesn't really explain why it starts and idles when you depress the throttle plate. That shouldn't effect fuel pressure from the pump surely ?

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Hmmm. A few thoughts:
Sure sounds like it wants more fuel, and the air valve normally controls that function (along with the EHA, particularly on startup).

A big 'ol air leak (like maybe the backfire blew a hole in the lower rubber cover under the throttle body) would require a lot more fuel (via the depressed air valve) but it would cause the engine to rev higher as it got more air via the leak and more fuel due to your finger on the air valve.

When you push the valve down to make it run, how fast does it run?

Does the air valve look like it's properly seated - if the pop was indeed a backfire, it could have damaged or unseated it.

Can you keep it running and check the vacuum gauge, should be pegged left if there's not a big air leak.

Might unplug the idle air control to take it out of the picture: it should rest partially open but stable when unplugged, so it doesn't change the air flow as you look for sources.

The EHA plays a big role in starting enrichment, but if adjusted properly doesn't do a whole lot during warm running conditions, so it may not be the culprit, just guessing.

A very rough pressure system check can be done by turning on the key a couple times, then pressing down on the air valve. You should get a noticeable resistance if there's fuel pressure. If you press and release it a few times you can feel the resistance drop as the pressure subsides. Then run the pump again, and wait a few minutes before pressing the valve, to see if the pressure holds. A good one will hold 30 minutes or more.

Thanks for the reply, I will check those things you mention.
If i depress the throttle plate more than just a crack it stops.
Open it too far it wont start.
Its almost as if the fuel system is getting a false reading and wont provide air to the system to start, butnis happy if you manually intervene but only up to a point.
I'll recheck the bellows later again to ensure its fit and integrity.

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SClasss i go along on the same lines as you . Thumbs up .

And i think may be -- the EHA valve .There is a way to check them. But the old foe the OVP relay is a problem area also check fuses on the top. Min problem is just as you are getting with your stop ,start , and stall .

PROBLEM SOLVED

Guys thanks for all your handy suggestions and tips.
Before I provide the solution to my issue might I ask how you tell if the EHA is still functioning properly ? Being a sealed unjt with couple o rings, i do detect a small seepage around the fa e that fits to the fuel distributor. The 2 o rings seem okay but not sure ifnit needs replacing as a unit.

Now without further ado, the culprit of my woes is
IDLE CONTROL VALVE INLET HOSE
Yes if you recall I mentioned it backfarted when I went to start.
It was through the throttle body notnthe exhaust and it blew the inlet hose connected to the ICVout of the throttle body. A friend came over to throw a fresh set of eyes and with a bit of contortion be could see the end sitting in the middle of nowhere. Five minutes to undo and refit.
BAM Ignition start and running.

So relieved I really thought itnhad to be something simple but what.
Now tommfind out what caused the backfart......

Thank you once again kind people



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Zender you can replace the o rings .But before you remove the unit mark around the outside so you know exactly how it goes back on .Fail to do this and it will need re setting with a d/ meter .

Glad you got it sorted easily. Was going to suggest a hose might have blown off. I have an old 1974 engine driven welder that backfires occasionally when you light off a rod and blows a hose off. Same deal. The great thing about these M103 engines is that they are pretty much 1940s technology from their simplistic standpoint. Even the injection system is pretty stupid simple, except the computer controls for idle and EHA.