Left and Right bank running rich | Ford Explorer Forums - Serious Explorations

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Left and Right bank running rich


New Member
March 8, 2010
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City, State
Vacaville California
Year, Model & Trim Level
1999 4x4 Explorer 4.0 OHV
1999 explorer 4.0 OHV (VIN X). I changed the head gaskets (#6 hole was blown), fuel injectors, MAFS, Spark plugs, wires, distributor, fuel pressure regulator, fuel filter. I keep getting a check engine light for P0172 and P0175, left and right bank running rich. The injectors were bought on ebay from a reputable dealer. They were also advertised as the correct injectors for the OHV engine, not the SOHC engine. When I checked the flow rates for the old ones and the flow rates for the new ones, the new ones turned out to be about 6 lbs per hour more than the old ones. Additionally, the old fuel pressure regulator was showing 65 psi at the rail and did not change whether the engine was running or when I snapped the throttle. It also didn't change when i disconnected the vacuum line. Buying a new fuel pressure regulator was a waste of $108.00 because nothing changed. The brand new one had the same exact characteristics as the old one. Could either the fuel pressure regulator be bad from Ford or the higher flow rate injectors cause the engine to run rich and illuminate that light or should I be looking at something else? Any help would be appreciated.

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returnless fuel system

In 1999 Ford Explorers went from the previous year return fuel system to the returnless system. The fuel pressure regulator that previously was mounted on the fuel rail was changed to a damper and the fuel pressure regulator was installed in the fuel tank. The fuel pressure on your vehicle should be about 65 psi at all engine speeds.

What was the flow rating of the fuel injectors you purchased on eBay and installed?

Have you replaced the upper intake gaskets? The codes you are getting can be those. The gaskets compress over time and lose their ability to seal, causing too much air to get into the system.

I checked a flow rate table on this link:


The old injectors were 0-280-155-962. The flow rates of those were 12.7 lbs/hour or 133.5 cc/min 45 psi. The ones I purchased online were 0-280-155-700. The flow rates are 18.25 lbs/hour or 191.8 cc/min at the same pressure. The 700's are the ones that come up on a search for injectors for this vehicle but it doesn't seem right if the ones that I pulled out were lower flow injectors. But I have found the 962's through kragen. To answer the other question, I used a Fel Pro gasket kit for the intire head gasket change. I am certain that there is no leak anywhere in the engine bay. I sprayed around with some carb cleaner and there was no attitude change. The engine runs great through all fases of operation except when at idle with it in drive. It feels like a small staggering missfire. Its almost unnoticable.

rich not lean

An intake leak would result in lean engine codes instead of the rich ones your PCM is reporting.

Your replacement injectors have a 33% greater flow rate than your originals. Maybe your vehicle was originally sold in California. I suspect the replacement injectors are the source of your rich DTCs. You could correct for the higher flow injectors with a custom tune or reverting to the lower flow injectors. If you have access to a scanner you could confirm that your long term and short term fuel trims exceed their limits.

I'll be damned. Good call on that. I bought the vehicle here in California where I am currently stationed. So the injectors are the correct injectors for this vehicle, just not a California vehicle. It's a good thing I'm moving to Illinois. A little closer to home where things are normal. Thank you for the info and I'll look into a tune instead of ripping the upper end apart again. What would be the cheapest way to tune it? Perhaps an ECM from a non-California Explorer?

Were the other injectors bad? If not, throw them back in and see what the vehicle does.

You said you changed the MAF also. How long ago? I few years back I recall those codes and if I remember the MAF was filthy.

Three of them were not servicable after they were removed. Three of the Plastic end pieces came off and one ended up in the cylinder. I only have three left that I kept. Both the MAFS that I have work fine. I changed the MAFS shortly after the codes popped up after the head gaskets were done. I wasn't getting a light before the head gasket change except for a misfire in cyl #6 which is where the head gasket was blown.