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Propane Camry with P304 fault

05 Camry 2.4L. ~ 250K hassle free miles. Runs on both propane and gasoline. This particular issue is not related to running on propane but hopefully I have your attention.

Issue: p304 misfire.

1- Checked plugs. Everything seemed ok. Replaced and properly gaped plugs. No change.

2- Checked for vacuum leaks. Sprayed all vac lines with starting fluid. No leaks detected.

3- Ruled out bad fuel injector by running the car on propane. The propane system uses a separate fuel injector system for injecting the propane into the intake runners. The propane computer piggybacks on the OEM sensors and will cut out the signal to the gasoline injectors and instead fire the propane injectors.

4- Swapped coils. Still # 4 misfire. Later replaced coil. No resolution.

5- Downloaded Torque app and started monitoring engine sensors. Noticed that during acceleration the MAF would periodically lose signal. First cleaned MAF using the MAF cleaner and later replaced the MAF. No changes. See screenshot below.

6- Continued to monitor the sensors and noticed that the MAF signal and vacuum signal would drop off. I later concluded that this appears to start happening ~ 2500 RPM. After some research, I noticed that the vacuum switching valve kicks in at 2500 RPM. I thought I had my answer. I replaced the switching valve (the one mounted on the air intake hose) with one at the salvage yard and I still have the same issue.

7- The manual i purchased does not have engine wiring diagram. It only has the stuff for power locks, windows, radio, etc. No sensor wiring diagrams. I need a detailed wiring diagram.

What are my other options? Need advise.

I have not checked the compression. I ruled this out when I noticed the sensor signal issues. I have always monitored the oil via oil analysis and never noticed any excessive wear metals in the oil.

I suspect there is a wiring issue on a wire that is running the vacuum switching valve or the MAP. Other potential issue is faulty computer.

Other maybe useful information but I doubt it. Car has 250K miles. Was converted to run on LPG @ 100K. Of the last 150K miles 90% of this has been on cheap and clean burning propane. For the past couple of years, I get a puff of smoke at startup. I have always assumed this was worn valve guides or valve seals. Likely guides since propane does not provide the same lube to the valves that ethanol lased gasoline provides. I typically change my oil at 15-20K mile increments and have sampled the oil and never detected any premature wear metals. When running on propane the oil is just as clean after 20K miles as it was when it was put in the engine. Maybe I will recycle this lightly used oil in my lawnmowers.

Other maybe useful information. I live deep in the sticks. I have had a problem with pack rats eating wires and hoses on my trucks that typically sit unused weeks at a time. People look at me strange when I use the vacuum at the car wash under my hood but not on the inside of the farm truck. Never had a issue in my cars maybe because the wires are not made with soy or fish oils like some wires are or maybe it is because the car is always parked at the house and my cat named Death Ray does a good job of exterminating anything that moves near the house. Here is the picture of the rat that I caught in one of my trucks.

Can you feel the misfire?
Idle smoothly?
Power reduced?
MPGs down?

yes I can feel the misfire. It idles rough. Acceleration is diminished. MPG is down ~20%.

This is true while running on both gasoline and propane. The noteworthy thing about the propane system is that it reads the signals from the gasoline injectors and mimics them on the propane injector. So the gasoline injector is not getting fed the proper signal from the OEM computer. I have narrowed this down to the MAF sensor loosing signal. If the MAF reads zero air moving into the engine the computer will tell the injector not to fire.

At least this is my current diagnosis. It maybe wrong but well thought out (at least to me). The problem is I don’t know how the whole system works together. Once again, I would disregard the whole propane setup and focus on as if it was just a stock OEM car.

Smoking at start-up sure points to worn valve guide seals.

The problem might be that oil from the worn valve guide seal on number 4 cylinder intake valve is the only one leaking or is leaking worse that the other valve guide seals. And now there’s deposits on the back side of the intake valve for cylinder 4.

These deposits can absorb some of the fuel entering the cylinder causing a lean condition and a misfire.

I would try adding a can of Seafoam to the gas tank, fill the tank, and run the vehicle on gasoline until the tank is used up to see if the Seafoam removes any possible deposits on number 4 intake valve.


But that does not explain the random null value readings that the OBD2 reader receives from the MAF and vacuum.

Is there another valve switch near the gas tank (some sort of charcoal canister) that connects the variable vacuum valve that is located at the air intake?

Do you have a MAF sensor code?

If not, focus on the code that you do have. Then once that’s solved, worry about the MAF sensor signal


I’ll go off topic

Did anybody else notice this . . . ?!

“I typically change my oil at 15-20K mile increments”

I admit I’ve never personally used propane on any of my own vehicles . . . but 15-20k intervals seem pretty high to me

oil does eventually break down, and I wouldn’t want to have the stuff in the crankcase, when that happens

I know this doesn’t even address OP’s problems, but I found it interesting, nevertheless

If you’re getting a misfire on number 4 and not other cylinders, a common diagnostic method is to swap the various components to a different cylinder and see if the misfire follows.

  • spark plugs
  • coil packs
  • ignition wires (if possible)
  • fuel injectors

If that doesn’t turn something up, then you’re looking at various fuel injection, electrical, and compression tests probably. You might be better off to take it to a pro who has the proper test equipment for a correct diagnosis, then you can fix it yourself if you like. You have one thing in your favor, it is only occurring on one cylinder. That eliminates quite a few possibilities.

Tester. I do not have a maf error code. When I search the p304 code there are several things that apparently cause it including maf.

The hesitation and rough idle does not happen all the time. It is periodically happening more like a electrical fault and not a mechanical fault.

I plan to hook up my laptop and read more detailed info than I can get on my smartphone.

If you can get a “fuel trim” measurement from you test set up, that can be helpful sometimes in a problem like this.

Db4690. Yes the long interval seems insane. I have been using oreilly house brand synthetic. There are synthetic oils that now advertise they aRe good for 15k. So on a clean burning engine it would be good for much longer. As I said in the original post, I send my oils out for sampling to check internal wear. The oil has always came back after 20k within all oem specs.

15-20k miles is only 3-4 months for me. I have a long commute.

George. I have already tried all the items that you recommended.

I originally ruled out the low compression as a culprit. After researching, these 2.4l engines had a high failure of head bolt threads. Maybe I should go back and do a test on the compression .

I think you need to concentrate on the #4 cylinder. MAF and MAP sensors affect all cylinders, they wouldn’t just affect #4 all the time.

I would suggest that you run on gas only while troubleshooting this. Then you can pull the #4 plug and check it for wetness or carbon build up. The clean burning propane could be masking the problem making it harder to find.

BTW, I would keep an eye on your coolant level and check the #4 plug to see if it looks like it has been sandblasted clean while in the cylinder. That is an early sign that you have a very small coolant leak into the combustion chamber. At first, the drop in coolant in the system is hard to detect as it uses so little, but it only takes a very small amount to cause a misfire and it shows up on the plug as a sandblasted appearance.

@“Propane Car”

The very first thing I’d do is a compression test, as you already mentioned

If it’s low, you need to follow up with a cylinder leakdown test, to pinpoint the cause

Unfortunately your engine does have that problem with the threads pulling out of the block. There are thread repair kits available, should that be an issue

I would measure resistance on all of the injectors. An injector balance test would also be a good idea

But you should rule out mechanical problems first, IMO

The weird thing about this is the misfire is periodic and intermittent. Once again this intermittent misfire led me down the rabbit hole of electrical. This issue first started about 7-8K miles ago. For the last 7-8K miles I have been driving on mostly on gasoline. I have only run a couple tanks of LPG for testing and troubleshooting.

Also on an additional note, I have tripped all the P300, P301, P302, P303, & p304 but the P304 is the predominate and consistent code that reoccurs.

I forgot my laptop at home so maybe I will use my work laptop. I can hookup a laptop to the propane computer which reads several things from the OEM computer. The key thing that I want to read from this is the timing of the fuel injectors that the OEM computer is signaling to the injectors. I am curious if the injectors are receiving a null signal from the computer at the time when the signal from the MAF goes null.

This problem has been a pesky nuisance. If i can simply hold off a few more weeks I will start commuting on one of my motorcycles and allow me more time to get the car in my shop for some more in depth inspections. Currently I have too many irons in the fire and I have to attend to the most urgent ones first which has not been this car. Luckily I have several other modes of transportation but non of them are as economical to drive.


Last night I did the compression check and 1-3 had 185psi and 4 only had 120psi.

After removing the plugs and going the compression test, I reassembled the plugs and coils and the car was running exceptionally rough. I checked the codes (wish I would have done that before removing the coils) and I got a new code I have never got before. P0354 which sounds like bad #4 coil. This coil is a new coil that I put on the car a couple of months ago. I still have the old coil that I took off. Maybe I will try swapping it back on and see if the same issue and code is present.

120 PSI compression is on the low end of functioning. I would really like to kick the can down the road about another month so I can more comfortably commute to work on my motorcycle while I fix the car but I still have other transportation that I can drive.

I think the plan is to pull the head and see what is causing the problem. if it needs a simple rebuilt head I will probably do that. If there are signs of significant cylinder wear or the potential bad head bolt threads, I will have to think about what I really want to do.

As I think about the cost of a new car, I think I can expect about $0.085 per mile in the cost of a car. If I can do a few repairs and get another 100K out of my car, as long as I spend less than $8,500 to do that then I would be financially better off. I would never spend that kind of money on a 250K mile car. There is nothing difficult about removing and rebuilding an engine but it just a major time suck.

@“Propane Car”

I kind of disagree with you about the compression

IMO, 120psi is low enough to be the cause of a misfire

Consider a couple more tests before pulling the head. A wet/dry compression test would tell you something about the rings, and a cylinder leak down test would tell you something about the valves and head gasket.

@georgeSanjose @db4690

I resolved the P0354 code last night. It was actually a bad #3 coil. I started it and it was running rough and pulled each coil harness wire from the coils 1 at a time. When I pulled #3, the car was running the same. I cleared the codes and a different code came up that identified #3 coil this time. I removed the coil and replaced it with the one that I originally removed from the car when I purchased a new one. The car started running the way it had been with slight hesitation and miss and the code reset back to the P0304 (#4 cylinder misfire).

The car is sitting at home while I come up with another idea. I may change the oil and send the oil off for analysis (will take about 1 weeks to get results) and that will tell me if there are contaminates in the oil.