Basic Info:
74 1.8 L-Jet
Just installed new SMP injectors
Fuel pressure gauge installed.

I installed new injectors on 1 & 2 yesterday to replace a leaking injector, and it ran really nice after - better than before, leading me to believe that the old injects weren't 100%.
So, today I installed the other 2 new injectors on # 3 & 4 on passenger side. Now won't start at all. Fuel pressure gauge shows 36. I checked all fuel lines, all injector connectors, all vacuum lines. All looks good. Still no start. Time to step away, go get lunch.
Hour later, try again - she starts but backfiring really bad through exhaust. Check all vacuum lines again, all look good. Start it again, starts good, but is running rough. Idle is way high, adjust idle screw as it warms up to get idle down to 900. I think I may need a new cold start valve. Once warm, steady idle at 900. give it a little gas, and hold steady around 2700 rpm and it backfires through exhaust. To see if a specific cylinder is missing, I unplug wires one at a time. Pulling #4 makes no difference. #4 isn't firing.
Plenty of spark at the wire. I THINK the fuel injector plug tested ok. Took out injector and pointed it into a bottle. Had the lovely wife crank it over, no gas going into bottle.
AHA - bad injector. re-install old injector. Now no start at all. Check engine bay, and the hose on the old injector has also sprung a leak. WTF!!!!

Also, when testing the fuel injector plugs with a test light, one side was always hot, the other side would be lit with key on, then flash when engine cranked over.

Deep breath. Reinstall the new SMP injector again. No Start. Sit down at desk and throw myself on the merciful wisdom of the club.
Help me Obi-Wan Kenobi's, you're my only hope (kids are watching Star Wars in the other room...).

36 fuel pressure seems high. I think it should only between 28 and 31 psi

:agree:not

the pressure in a 1.8 is 2.5 bar + or - 0.2 bar.
or around 36 psi. so its about right at 35 maybe.

the proceedure for testing is in the factory manual.
you have to have a few things disconnected and operate the flap in the airflow meter with the ignition turned on. that pressures up the fuel pump. not sure you are necessarily testing the fuel pressure reading correctly, but ....

hmm. have to think about the rest of it you are describing.
someone here will know.

Check all the vacuum hoses and harness connections something got knocked loose or disconnected. If everything looks good, check that the flap closes in the air flow meter, as you stated it was backfiring. Last resort, remove the harness and check continuity from the main plug and 12 pin connector to the connectors. Jeff Bowlsby has diagrams telling what pin goes where.

the full test for fuel pressure is to disconnect the vacuum hose from the pressure regulator. the vacuum hose is easy to get at with the aircleaner out of the way. the whole thing is easier with the aircleaner apart anyway so you stick your fingers in to operate the intake flap.

turn the ignition key to on with the intake flap pushed open. you should get 2.5 atm/35 psi.

you can do a test with the engine running if the engine runs.
let the engine idle. disconnect the vacuum hose. (sometimes easier to disconnect the vacuum hose at the intake plenum if the air cleaner is installed - though you got to get your fingers in there as well and its under another hose from memory)
should read 2.5 atm. 35 psi (a bit either side ok).
reconnect the vacuum hose, should drop to 2.0/2.1 atm. 28 psi - 30 psi.
rev the engine, pressure should rise.

if you were running the engine at idle when you read the pressure with the vacuum line in the pressure regulator, 35 psi is too high.

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i'd be testing the injectors with some swapping over of 3 and 4.
just to make sure its not the injector 4 itself. take 3 out and swap it for 4 and make sure it sprays vise versa. you can start to narrow it down to maybe something in an electrical plug connector or wiring.

it sounds like it must be a plug or connector that is loose, dirty etc.
the car ran fine before you changed the injectors?

Check your seals on the injectors.
I learned the hard way that the SMP injectors dont have the same size large retainer rubber thingy. Too small and doesn't push injector in enough.
I then learned that if the small injector seal into the manifold doesn't look fully seated, it aint.
Good luck

I caught that the SMP big o-ring was too small and pulled through the bracket and re-used the old ones which worked well. I thought that he smaller seals were pretty tight, but there is no real way to tell because you can't see them. If the small ones aren't sealed well, will it cause issues like I am experiencing?
Thanks

No start and back fires were things I also experienced.
Try to get your finger down there to feel if rubber is not flush with manifold, or try a mirror to look around.

Thanks for the tip on the injector seals.
I ordered a good set of seals (the SMG ones are questionable) that will be here tomorrow. I also ordered a bunch of new vacuum line, cold start valve gasket, intake manifold hoses, and intake manifold gaskets. Hopefully one of those will hit the mark.

you may as well check your throttle body gasket (body to plenum).
i discovered mine had gone hard with age and was leaking air when the car warmed up but not when it was cold. a good samaritan 914 owner here sent me one. you can get them from 914 rubber.
it was enough to really upset the car. but its not your problem by the sounds of it at present.

there are some really good articles on The Samba about L Jet. You can find information for every single pin and part and how to test each. They also include charts of symptoms and where to look first.
When I got my 914 a few years back the only thing that worked was going through the list one by one and ticking off possibilities. When I recently built the big motor for my NotchBack I used the articles first to make sure I was getting the readings I needed at every point then started tracking down each part from there. My AFM was not even calibrated, I made it from several, so I was starting from Zero on a big motor. Eliminating one possibility after another winds up taking less time for me than shotgunning the approach.