doruk

Hello, I bought a 1989 model jeep cherokee xj 4.0 automatic in recent months.
The vehicle has LPG fuel system.
The car runs smoothly with the LPG system and I go long distances this way, but it stops working after 3-4 seconds on gasoline.
After disconnecting the map sensor hose from the throttle, the car idles, but when I press the gas, it shakes and stops working.
Changes made on the vehicle MAP sensor has been replaced, TPS sensor has been replaced and it gives 0.82v output when the gas is not pressed, spark plugs have been replaced, oxygen sensor has been replaced, fuel pump has been replaced, pressure regulator has been replaced, fuel pressure is in the range of 3.5-4 bar. fuel injectors were disassembled and cleaned and no problems were detected (equivalents were bought from aliexpress but the originals were reinstalled when the same problem occurred)
I found a topic with this problem, but the owner of the topic did not share the solution he found, thank you very much if you can help.
The lpg system was installed with almost no intervention to the engine of the vehicle. There is a breaker for the injectors in the system and there is a breaker that cuts the electricity to the fuel pump. It gives lpg to the engine through the hose at the inlet of the throttle. I don't think there is a problem with the gas system because I was suspicious and canceled the system completely and the problem was the same.

Saudade

Fuel pressure sounds high. 1989's should be in the range of 31-39 PSI (2.13-2.68 BAR). With the breakers set to use gas, when you turn the key to RUN (not START), you should hear the fuel pump run for a few seconds and stop. This primes the fuel system. When you turn the key to START, how fast does the engine crank over? Is it slow/sluggish or does it turn over briskly? What are the battery voltages (off, Idling while it can). The Renix ECU needs to see about 300 rpm before it starts firing the injectors in sync with the spark plugs.

Since your XJ runs well on LPG, then spark, timing, compression are OK and the sensors used to input to the ECU, so you issue must be within the fuel system I think.

All fuel injectors share the same ground through a large splice in the main harness alongside the valve cover. If you have a multimeter or test light, you should confirm a good ground on all injectors. The other lead is the 12v feed from the ECU (located under the dash). The ECU fires the injector in sequence by sending 12 v to them. You could also use a test light on the 12v lead to each injector and see if it flashes while cranking.

Can you post a picture of your engine bay and then a few closer up of the LPG components?

doruk

My vehicle has an adjustable valve for fuel pressure, as you said, I reduced the fuel to the range (2.13-2.68 BAR) and there was no change. the fuel pump works continuously in gasoline mode without stopping and keeps the pressure constant. the starter motor turns at a satisfactory speed and can turn the engine without much difficulty. battery voltage is 12.5v when the vehicle is stopped, 14-14.5v when the vehicle is running.
After testing what you said, I saw some loose connections in the injector cables and I completely disconnected it from the vehicle to check the electrical wiring and started to check it. In 1-2 days I will check the loose connections and put it back in the vehicle.

There is one thing I am curious about, I disconnected the engine cables from the headlight part of the vehicle with a socket, is there a system such as a relay or button that could cause such a problem in the rest of the vehicle?
I am having trouble uploading pictures to the site, the pictures are not loading.
I am writing my contact number below, if you wish, I can send you the pictures via whatsapp.
contact: +905446560302

Translated with DeepL.com (free version)

doruk

Good morning,
I have carried out a full examination of the electrical wiring on the engine side.
Following a thorough examination of the electrical wiring, I identified a minor issue with the fuel pump relay connections. I promptly rectified the problem and now, upon turning on the ignition, the fuel pump initiates and pauses for approximately two to three seconds, as intended. However, the underlying issue persists. I have meticulously reattached all loose connections with solder and provided comprehensive insulation.
I am beginning to suspect that the engine control unit may be the source of the problem.
Upon opening the engine control unit cover, I observed that two of the integrated circuits were slightly warmer than the others. I will attempt to upload images of the unit for your reference. Do you believe this temperature discrepancy is within normal limits?

doruk

Good day.
In accordance with the electrical schematic, each cable was examined individually to ascertain its point of origin and destination.
It was observed that two cables were absent. Could anyone shed light on the function of these cables?

(1) up shift ind A8 named cable located in the brain socket
(2) tx serial data out D12 cable in the brain socket

Saudade

The "Upshift Indicator" is for manual transmissions. It simply lights a dash indicator. Since yours is an automatic, there's no need for it. Note, one branch runs to the cluster. There is likely no bulb in that cavity.

The TX Serial Out goes to the Diagnostic connector. It's for connecting a DRBIII scanner or equivalent (like a Snap-on MT-2500 with proper connectors and cartridges).

https://nickintimedesign.com/ also makes a Renix diagnostics tool. Don't know if he still does.

doruk

Despite my best efforts, I have yet to identify a solution to this problem. I am curious whether others have encountered a similar issue and, if so, whether they were able to find a resolution.

dmoe69

You have a rather unique set up, there aren't a lot of subscribers with expertise with it. I am curious though, does it run for 3-4 seconds when already running and you switch from LPG or will it also start and run for 3-4 seconds when started in gasoline mode?

doruk

In petrol mode, the engine runs for about 3-4 seconds before it stops running.
When the engine is running smoothly in autogas mode, the fuel consumption is significantly higher, about five times that of autogas compared to a typical 2000cc passenger car.
When the map sensor is disconnected, the engine stops running completely.
However, when the map sensor hose is disconnected from the manifold, the engine maintains a constant speed in petrol mode.
However, the engine stops running again when the accelerator pedal is pressed.

dmoe69

You could try back probing the B8 pin of the ECU to ensure it is getting correct voltage through the LPG/gas switch (I have no idea what that looks like, or how it does that). It would also be good to verify you have good sensor circuit ground by back probing from the "B" terminal of the TPS connector to the negative terminal of the battery.