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Lean codes off idle

  • Wood
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13 Sep 2020 02:21 #43024 by Wood
Lean codes off idle was created by Wood
I'm just curious about what the course of action would be, hypothetically, if you had a car come in with lean codes off idle. You look at fuel trims and they only get worse with increase in RPMs. You start to focus on either a MAF or fuel pressure issue. You go on a road test and you notice that you get over 4 volts on the MAF signal when you wind it out. So you rule out a faulty mass air flow sensor. You get back to the shop and you hook up a fuel pressure gauge. KOER you have good fuel pressure but you notice when you shut the engine off you start to bleed off fuel pressure. This is a mechanical returnless fuel system. Your next step might be to pulse each injector and check for fuel pressure drop to see if there are any restricted injectors but you cant if the check valve in the pump is bleeding off pressure once the fuel pump turns off. Do you tell the customer they need to spend costly money in replacing the pump before further diagnosing the vehicle, or what is the call on this one?

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  • Tyler
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13 Sep 2020 14:50 #43039 by Tyler
Replied by Tyler on topic Lean codes off idle
I can't say I've ever run into that particular combination of faults before. :silly:

Answers will vary, of course, depending on the shop, the technician, and the specific vehicle. How hard is it to get a fuel pressure gauge installed? How much struggle is it to dig the PCM out? If it's easy breezy, I'll continue for the initial diagnostic fee. If it's a fight, I'll sell more diagnostic time.

The fuel pressure kit I own includes a shut off valve that can be installed wherever needed in conjunction with other adapters. Again, depending on the vehicle, I'd install the gauge on the rail side of the shut off valve. Energize the pump, get the rail pressure up, then close the shut off. Pulse the injector and record the reading. Repeat for the other cylinders.

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13 Sep 2020 16:27 #43042 by Matt T
Replied by Matt T on topic Lean codes off idle
Did you, hypothetically :lol: , take it back out for another road test with the pressure gauge attached to see if fuel pressure dropped with increased load? Or at least try increasing RPM, or power braking, in the bay?

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20 Sep 2020 16:13 #43277 by Flatrater
Replied by Flatrater on topic Lean codes off idle
Hello Wood

Some random thoughts...

Perhaps this should be restated, fuel trims increase with airflow.

MAF sensor output is not linear due to the sensing device being used. The response is somewhat flat (linear) between 10-60 g/s. Beyond that, it changes exponentially and loses precision.

The last thing I want to check is fuel pressure on most vehicles. Especially Asian vehicles. So it may be necessary, but I'd perform others tests first.

Given your scenario, I would make two test drive captures. Idle, light cruise, cruise, and WOT. Before the second run I'd clear trims. That will be important in assessing the MAF. LT learned corrections are usually used in the upper range and WOT.

That just ruled out most of the scan tools on the market... What a shame. It is such a powerful technique.

I would then sit down and analyze the captures. Wait... if this is a 2008 or above vehicle, I would also perform a WOT run and record (only) the ABS_LOAD pid using generic data. Peak VE occurs (and corressponds to) at peak absolute load. (MAF diagnostic tip)

I would not rule out a MAF based on your voltage. All that did is tell you what voltage was achieved. In the ECU, voltage is used to extrapolate airflow based on flow numbers determined during design. It assumes a number of things.

I don't see how an injector drop test is going to help here. If you have a flow issue, it is (according to this scenario) an issue at higher flow rates. The drop test is a really gross failure test anyway, barely useful at times even with hard faults.

Fuel pump pressure bleed off at shut down is common and usually nothing to worry about. When it not only bleeds pressure but drains the rails, now that is a problem but usually only affects starting. Easy enough to check.

Your question kind of hinges around fuel supply issues at speed. You can rule that out (or in) with the test drive.

There can be times where you have ruled everything else out and need to either replace fuel injectors or have them flow tested. I had one a few months back where someone decided it would be a great idea to replace fuel injectors with the incorrect flow rates. The owner SWORE the injectors were original. Later... he remembered that he had inherited the vehicle from his dad who had passed away and wanted more power so he bought a set of "high performance" injectors.

He should have just left it alone... :-)

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