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My 1996 Cherokee ltd 4.0l (with an lpg conversion) running…

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my 1996 Cherokee ltd 4.0l...
my 1996 Cherokee ltd 4.0l (with an lpg conversion) running intermittently badly. chokes coughs backfires and then if I pump the fuel it goes again sometimes runs well. idles inconsistently. It has near new plugs leads...entirely new distributor because local Chrysler dealer told me he couldn't get me the pick up sensor. crank sensor and o2 sensor are but a few years old. have done a carby cleaner vacuum leak test and found nothing (but it is intermittent ...so would I?) I guess ignition coil is next on my parts to replace list. Should it be?
Submitted: 6 months ago.Category: Australia Car
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Answered in 14 hours by:
10/16/2017
AU Auto Mechanic: indy-tech, Australia Car Mechanic replied 6 months ago
indy-tech
indy-tech, Australia Car Mechanic
Category: Australia Car
Satisfied Customers: 7,347
Experience: All import and domestic car repairs and diagnostics
Verified

Hi, I'm Greg (Indy-Tech).
Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP.
Rgds Greg.

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AU Auto Mechanic: indy-tech, Australia Car Mechanic replied 6 months ago

Hi Justin .

Is your problem both the same on Petrol and LPG or just when you have LPG switched ON and runs good on petrol.

OR both system have this intermittent problem .

Rgds Greg

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Customer reply replied 6 months ago
on lpg there are momentary losses of power but not the backfiring
AU Auto Mechanic: indy-tech, Australia Car Mechanic replied 6 months ago

Is the check engine light ON ,Have you checked for malfunction codes

Greg

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Customer reply replied 6 months ago
No I havnt lately. Engine light has not come on since last time I ran it on gas maybe 3 months ago. When I do check the codes they are usually full of injector fault codes which are expected.
Last night I blew some smoke into the intake manifold and found a) pcv elbow grommets are not great b) the other end of the hose from the fromt breather did not seal on the barb c) a suspicious wisp of smoke from lower down near the base of the throttle body. i then put a cable tie around the hose and tightened the throttle body bolts that i could get to. Front left one moved nearly 3/4 of a turn. could not get a tool on the back let one to confirm it was tight. Car ran much better today although it still spluttered a little bit. much better and no backfires. I guess i am gonna need some sort of offset spanner for the back one?
Customer reply replied 6 months ago
Sorry scratch what I said about going better. just picked the kids up from school and had to waggle the throttle just about the whole way. I am so disappointed but I think back to scratch
Customer reply replied 6 months ago
i will put some gas in it in the morning and see how it goes on gas
AU Auto Mechanic: indy-tech, Australia Car Mechanic replied 6 months ago

Do you have a scan tool with live data .

When was the last service ,like spark plugs replaced etc.

Are you at the moment running just on petrol or both .

Is the LPG system MPI injected or Single injection at the throttle body .

All vacuum leaks have to be repaired PCV replaced .

Greg

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Customer reply replied 6 months ago
Single injection gas system. After driving on gas today I guess the answer to your question is that there is no sign of the issues I am having on petrol. Seemed to drive fine without incident and idles consistently. I do not have a scan tool but I followed instructions in the manual to extract the codes. I replaced leads and plugs maybe 6 months ago. When I installed the new distributor I noticed some oxidation around the post to cylinder 1 so I took out the spark plug but it appeared to be fine as it looked like the picture in the manual that said it is firing correctly. Fuel pump and filter are both about 8-9 months old but as I understand it the petrol tank is not the original and does not have the internal baffles to ensure the pump remains submerged. So it is possible that it needs replacing again but I do think it is too soon to expect.
Let me ask you a specific question. I purchased (via ebay in the US) some after market injectors. But they have 4 holes not 1. How long do I need to keep the car running before the computer will adjust the pulses for the new capacity injectors? Because I did install them but was then forced to take them out and put the old ones back because the car wouldn't run for long or under load.
AU Auto Mechanic: indy-tech, Australia Car Mechanic replied 6 months ago

Hi.

Okay so on LPG it runs good .so its for sure a petrol delivery issue or fuel air mix .

First attach a fuel pressure gauge to the fuel rail just behind the throttle body and check fuel pressure .

with the engine running you should have a reading of 47-51psi .

Make sure you did not use BOSCH spark plugs .

Injectors sound like you most probably got the HP engine injectors management system will not adjust to injectors.

Greg

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Customer reply replied 6 months ago
ok sorry took me so long. Gauge was just a tyre pressure gauge pushed against the relief valve, so I would not take the readings as gospel but with ignition on I read 10psi, with engine idling I got 46 psi. I also measured the vacuum pressure from the suction tube at rear pcv elbow at -65KPa (9.5 psi)
AU Auto Mechanic: indy-tech, Australia Car Mechanic replied 6 months ago

I will have to opt out and let a fellow expert help you .

Greg

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AU Auto Mechanic: sprinkles08, ASE Certified Technician replied 6 months ago
sprinkles08
sprinkles08, ASE Certified Technician
Category: Australia Car
Satisfied Customers: 23,800
Experience: ASE Master and Advanced level certified. Factory trained with 17 years dealership experience
Verified

Hello and welcome to JustAnswer!

What you're describing is often caused by a weak upstream oxygen sensor but there are many possible causes for what you are experiencing and it will require some testing to determine what is going on.

What method did you use to check for fault codes and exactly what did you see?

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Customer reply replied 6 months ago
the manual describes a sequence of turning the ignition on and off and changing gears that forces the check engine light to flash the stored codes. the numbers it flashed were 27, 27, 27,27, 51, 21, 56.
I have not done this lately and have since then had a new oxygen sensor installed and codes cleared.
would I be wrong in thinking that 10psi at the rail with the ignition on means my fuel pump is shot? I have repeated it many times. I will check the current codes and get back to you.
AU Auto Mechanic: sprinkles08, ASE Certified Technician replied 6 months ago
You'd need a fuel pressure gauge for accurate results.
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Customer reply replied 6 months ago
Sorry guys I think it is me that has to opt out now. I guess the point of this exercise was to iron out some issues with the engine before I spend some $ on the suspension so I can tow my camper over the mountains over xmas on a 40 deg day...(again) without having to switch to gas for the big hills to avoid stumbles. I think I am already there. I stupidly filled the petrol tank and I am running it down to enable me to open it and look at the pump and it has run well and without issue for 2 days I can only think I have unwittingly relieved some sort of vapor lock situation by messing with the relief valve. I thank you both for your expertise and apologise for having such an old bomb of a car with most likely multiple issues that I am sure will be an ongoing project for as long as it is on the road. it is good to know you guys are here to call on when these things come up. .
AU Auto Mechanic: sprinkles08, ASE Certified Technician replied 6 months ago
Good luck with it!
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