Z24i Engine Fuel/Mixture issue

[Killermiller]

Hi to everybody,

my newly installed Z24i has some fuel-related issue. It hardly starts, stalls immediately at idle, stalls when keeping the gas pedal depressed. The only way to keep it running, is to bleep on the gas continuously, and, even doing so, it is impossible to go past 2000 rpm.
When in limp mode, runs apparently fine (except for the 2.800 rpm limiter, of course) and idles well. This should narrow a little the possible causes list, because at least TP sensor, injectors and almost everything ignition-related should work fine (otherwise it won't run on limp mode neither, I believe). I've replaced the MAF sensor with a known-good one with no difference in behaviour.
I've looked for some vacuum leak letting some unmetered air into the manifold, but didn't find any. I suspect a bad O2 sensor, although it shouldn't give such a drastic symptom (the car is absolutely undriveable).
The car has an LPG system installed by one of the previous owners, and it exhibits the same problem when running on LPG, except that it will run fine if I clog the intake duct with a rag (hence my vacuum leak theory). Nonetheless, the rag trick won't work when switching to gasoline. 
I didn't perform an ECU self diagnosis, yet (which will do perhaps tomorrow). In the meanwhile, any input would be greatly appreciated.

[Slartibartfast]

I'm with you on the vacuum leak, especially if it's been messed with for the LPG conversion. How do the spark plugs look?

 

You can unplug the oxygen sensor for testing to see if the truck runs better without it. If it does, replace the oxygen sensor.

 

 

[Killermiller]

Absolutely no difference with the O2 sensor unplugged.
Also, I did perform a self-diagnosis and 1) apparently no codes are stored in the ECU, and 2) no codes are thrown during real-time self diagnosis (mode 5); I did not drive the car, but I was struggling to avoit stalling by depressing continuously on the gas, so I guess this should be enough for the ecu to detect a malfunction. This puzzles me, but also MIGHT indicate a vacuum leak too, since in such case everything should look fine to the ECU, only the ECU itself is unable to correctly provide the right amount of gasoline needed for a correct stoichiometric ratio.
I didn't look at the spark plugs, but,  judging by the exhaust smell, it seems running rich to me.
I've seen on the manual and there is that valve inside the throttle body, a V.C.V. valve (vacuum control valve?) that might be worth taking a look at.
In the meanwhile thank you, I'll keep you updated.

[Slartibartfast]

No codes meaning you got a 55, or nothing at all?

 

The color of the plugs might tell you if the problem is too much fuel or not enough. I'd also try unhooking the vac line from the EGR valve to see if the engine runs any differently with that disconnected. The EGRC solenoid on my dad's F150 was seized and the truck barely ran with the EGR open all the time.

 

Also, you say the engine was recently installed. What happened to the last engine?

[Killermiller]

Yes, three times a 55 code.

I've finally bitten the bullet and swapped the entire throttle body, including sensors, injectors and everything, with the one taken from the old engine. This solved the problem but also leaves some questions unanswered, which I don't like, but hey... 

Now the engine runs smooth and fine, without missing a beat. I had to enrich a little on LPG, though. I will go on with the restoration during this summer and will put it back on the road in autumn; by then, I will be able to perform a good and long test drive.

About what happened to the last engine, here is the (horror) story: 

 

 

Edited July 11, 2018 by Killermiller

[Slartibartfast]

Right! Forgot about that one. Good to hear it's running right with the old parts. Definitely irritating to not know which of the left-over parts are usable and which were the problem, but at least now it's not holding up your build!