I am having this low RPM problem with my Bronco:

It idles fine and engine runs smooth and quiet. But if I press gas to increase RPM while idling, I found the engine start to hesitate around 3000, it may go above 3000 for a short time, then come back below 3000. And if I press it again, it will never exceed 3000. It also sounds rough around 3000.

As a result, it's hard to drive it from a full stop because it affect transmission shifting.

Here is the scan result:

Key on Engine off:

334 - EGR Valve Position sensor: I don't think this has anything to do with my problem.

Key on Engine Running:

311 - Thermactor Air System problem - no operation during Engine Run Self-Test (Bank #1): This may be the problem but I have no idea what it is...

538 - Insufficient RPM change during Dynamic Response Test, or Invalid cylinder balance test - throttle position movement. or Invalid cylinder balance test - cylinder identification problems: This one completely confused me...

If "insufficient RPM change" means I didn't go beyond 3000 even with full throttle, it's right because I can't do it! But it didn't tell me why I can't do it.

Could anybody help me and tell me what may be wrong?

I have a new fuel pump and fuel filter so I don't think there is anything wrong with fuel delivery...

Thanks!

 

check fuel pressure when the condition exists, this will confirm fuel pump or not.

Did you replace with Ford fuel pump? If you just replaced the pump and not the whole body then this might be your issue, there's a check valve in the body that goes bad and causes the issue you describe.

 

An autoshop (Goodyear) replaced the fuel pump/filter about 4 months ago. I didn't have any problem until now. I also just replaced the filter today to make sure.

I don't have the tools to check fuel pressure and can go back to the shop to let them do it.

The code makes me feel there is something wrong with the air/exhaust...

Thanks!

 

Some updates:

Since I can't identify the problem, I took it back to the autoshop again this morning.

And they just called and told me after they removed the whole exhaust, the truck runs fine!

So some kind of restriction inside the exhaust system caused my problem. I have a strong feeling it's the cat converter because I just can't imagine anything else can cause that much restriction...

 

More (and last) update:

The shop called again and told me it's the rear cat converter. It costs $500 to replace it. Then I asked the condition of the other one and they don't really know. So I told them to do both because I can't trust the other one either. I hope $800 will fix this forever.

 

Yes, a plugged cat will cause that. Did your exhaust at any point smell like rotten eggs? that would be a clear signal.

 

wow, that is way over priced. hi flow cat from magnaflow is less then $100.

 

Honestly, it sounds like they are quoting you OEM replacement numbers. Skip it... follow Kem's advice.

 

Code 538 - did you snap the throttle when the system asked you to during the koer test ? If not that's where that code came from .

 

$100?? I wouldn't believe it if I hadn't seen thisy here:

Exhaust Systems Direct - Search Results for "94007 93307 93516 94016"

 

I think 538 only tells me something I already know: the RPM can't ramp up above 3000 doesn't matter how hard I try...

And I think 311 can tell something about this problem: no air because no exhaust.

I know the price is high because this shop charge $92/hour for labor.

If the part is $100/each and I need two, they spent 2 hours just to find the problem and 2 hours to replace them, that's $600 already, right?

Since I can fix most of the problems on this car myself and only leave the most difficult ones to them, I don't blame them too much if they charge me their book price, especially if I can save other people here some money in the long run...

Thanks every one!

 

the thing is, you only need 1 cat. if both are clogged and past there known life cycle the new hiflow cats are meant to replace the dual cat system of your year truck. I know this because thats how mine is setup. I paid $350 for my complete exhaust system build from the headers back all 3 in, custome y pipe etc.. I did provide the muffler.

I wish i had seen this post sooner, could have saved you some coin.

 

I will double check to make sure they put two under there when I go to pick it up tonight. Otherwise, I will ask for half of my money back for sure... :-)

This shop is the closest one to my house and that's the reason I keep using them. Since I can fix most of the problems on a car, if I have to use them, that usually means the car is in very bad shape and can't drive too far...

Since their "diagnose" fee is also $92/hour, even I don't fix there, I have to pay them $200 for the two hours they spent.

I should start looking for another place around here.

Thanks!

 

an exhaust shop charges far less to do exhaust work then a regular shop. thats probably why your getting bent over. Sorry

and based on your thread title, i would have told you to unbolt your exhaust at the manifolds and you would have noticed it reving properly and clogged cats would be the culprit. would have taken less then an hour. if it took that shop 2hrs to figure that out its for sure time to go elsewhere.

 

I just picked up my truck. They did replace two converters with OEM ones and told me the parts cost $500.

The truck drives like new now because I already forgot how it should drive like. With the converters gradually clogged up, I just didn't feel anything wrong with it before.

BTW, anybody fixed this "334 - EGR Valve Position sensor" before? I probably should just buy a new EGR. But I don't even know where it is...