01 CHEVY EXPRESS bad HPR?

[CNGCARS]

Hello,
I have a customer who has a 01 express bifuel. A few weeks back he brought it to me because the it kept switching to gasoline mostly on the freeway. He told me he had heard a loud pop a while back when it first happened. The van had a check engine light on. I scanned it and found a bad Maf sensor, I opened the hood and saw the airfilter cover was off. I assumed this was the pop. I replaced the air filter cover and cleared the code. A few days later it the check engine light came back one. This time I went ahead and replaced the maf sensor and cleared the code.
a week later the van poped the air filter cover several times. My customer zip tied it down and it still poped off the air filter cover. No check engine lights are coming on. This only happens when running on CNG on gas he has no problem. Am I right to assume the HPR is bad?

[siai47]

Re: 01 CHEVY EXPRESS bad HPR?

Usually a backfire (pop) is from a lean condition. Freeway driving, of course, requires higher continous fuel flow. I would check the inlet filter in the HPR before condeming the entire regulator. The '01 van is basically a mechanical system with a couple of solenoid valves for fuel control and idle control. These valves trim vacuum levels in the LPR. Failure of one of those should give you a DTC, so I would go to the HPR filter first. Also check the condition of the ignition cap, rotor, wires etc. If the van has a lot of miles, you might also want to check the ignition timing. These engines are known to wear the distributor drive gear resulting in retarded timing prior to gear failure. A scan tool needs to be used to set the timing as you are not actually setting timing in the usual sense. You are indexing the position of the rotor to the cap by comparing a sensor in the distributor to the crank position sensor. If the cap is mis-indexed, crossfire in the cap will occur. If the timing is retarded, you should pull the distributor and check the gear. This isn't a lot of fun as you will find the CNG components restrict the removal of the distributor .

[CNGCARS]

Re: 01 CHEVY EXPRESS bad HPR?

I forgot to mention I already changed the HPR filter. I have notified my customer we are most likely going to have to replace the HPR. Thanks if anyone has any other ideas please advise.

Thanks in advance.

[siai47]

Re: 01 CHEVY EXPRESS bad HPR?

If you remove the engine cover, there is a test port on the back of the engine where you can check the pressure coming from the HPR while driving the vehicle. Should be between 130 and 225 psi, however I have seen some under 100 psi and the vehicle runs just fine. When the cover is off take a good look at the plug wires, cap and rotor .