1999 Mazda B2500 (same as 99 Ranger 2.5 L), with an automatic and 210000 miles. Starts-up good and goes to high idle fine and running smooth. When the RPM's start going down, there's a minor knocking sound that I can start to hear, then as the RPMs drop a bit further, the knocking gradually gets louder until it sounds like a rod knock and the idle is rough and surging up and down, until finally it starts to warm up a bit and the idle smooths out and the knocking slowly fades away until it can't be heard. I wouldn't even mind this so much, except like clock work, after 3 start and drives, it'll trigger the check engine light MIL and when I scan it, there's a P0171 lean bank one code. I've done a whole bunch of stuff, (one thing at a time to see what happens), but still no luck and the cols start warmup knocking is still there.

Knocking during warm-up and pings like crazy at mid throttle...
Timing belt is new, runs between 5 to 7 degrees advanced when warm.
Changed plugs, wires, coils. Runs better.
Changed both o2 sensors, no change.
Changed oil and filter, no change.
Changed PCV valve, no change.
Checked hoses and connections for vacuum leaks, all ok.
Started running Premium 93 octane gas, pinging is completely gone and engine runs way better.
Changed MAF, air filter and reset computer, runs better but still same issue at startup.
Changed intake temp sensor, reset computer, no change.
Changed TPS reset computer, and that improved throttle response.
Changed IAC reset computer and that improved steadiness of idle at all temps and RPMs.
Changed fuel filter, and filler neck / cap, no difference.
Ran fuel injector cleaners one tank at a time, (Royal Purple, Sea Foam), no difference.
Evac/recharge and re-oil AC system. Made engine a bunch smoother and less RPM change with the heat, defrost and AC on.
Had exhaust shop inspect system and they repaired a couple of minor leaks on the mid pipe.
Ran Octane Booster, (engine ran a little smoother and seemed more responsive).
Went back to just Premium 93 octane with Seafoam, lost the smooth responsive feel, other than that, no change.
Ran Seafoam in crankcase and changed oil again, with half a bottle of Lucas. No change.
Performed Seafoam Top Engine spray procedure... Wowee zowee big difference in smoothness of idle and throttle / power during driving.
Seafoam spray worked so good, tried it again. No change.
Removed PCM and spray cleaned with contact cleaner/conditioner.
Removed, the PCM, cleaned with electrical cleaner and then went on to clean and reconnect every plug, ground, fuse and circuit. Runs the same.

So, to sum it up, I've got a 1999 Mazda B2500 with over 200 k that runs smooth as butter when its warm, (or cold while driving), and it gets 20 mpg. Its got a slight, very minor rough idle when stopped, in gear and holding the brake. The engine is so clean you can work on it without dirtying up your arms, sleeves or hands. It doesn't leak a drop of anything on the ground. All in all I'm pretty pleased with the way it runs and the fuel mileage. It idles smooth, takes off fine and when foot is planted to the floor it pulls right up into the 6k range without a complaint. The only problem I'm still trying to fix is this darn knock and lean code that occurs during a cold start / warm-up.

Any suggestions would be very much appreciated.

Thanks!

 

Welcome to the forum

Pinging /knocking is pre-ignition which means air/fuel mix is igniting from being compressed before spark plug can ignite it
This can happen if cylinders are running too hot, if spark is not advanced enough or if air/fuel mix is too lean

EGR(exhaust gas re-circulation) system was added to engines to reduce the heat in the cylinders, this lowers the production of NOX, a toxic gas that forms as cylinder temps spike when ignition occurs, the exhaust gas's slow down the ignition flame front spread so reduces the peak heat value.
EGR system is not used until engine is fully warmed up, since NOX very little gases would be created until engine/cylinders are warm
As a by product of EGR systems being used the spark could be less advance(to head off Pinging /knocking) to get better performance and MPG running 87 octane

Octane rating is a heat rating, 87 octane will self ignite at a lower temp than 89 octane, which will self ignite at a lower temp than 91 octane, ect......

Since Pinging /knocking occurs on warm up I can't see it being heat related, so that points to Lean condition, as does the P0171 code

The computer has no Fuel Pressure Sensor, 1998 and up Rangers use 55-60psi fuel pressure, computer is programmed to expect 55psi fuel pressure, so it "knows" how much gasoline it is adding to the engine if it opens an injector for 100milliseconds, for example.
This is the MAIN part of the computers calculation, it knows it is running a 2.5 LITER engine, so knows how much air will be pulled in at any RPM, it has the MAF sensor to confirm that and to adjust it for elevation above sea level, i.e. thinner or thicker air.
So the calculation it does is for "how long to open each injector" to add the correct amount of fuel to the air flow to get 14.7:1 air:fuel ratio entering the cylinders

Computer then gets Front O2 sensor voltage to tell it if exhaust is rich or lean
If Rich then computer reduces the open time for each injector, if Lean it increase the open time for each injector

Lean code is usually an air leak, MAF sensor is being bypassed so giving the computer the wrong info
But on the other side of that is Fuel Pressure, if fuel pressure is 25psi, then computer wouldn't be adding enough fuel, less fuel is flowing out of each injector, so Lean code becuse computer is having to open injector longer than it calculated.
Lean or Rich codes are not set until computers calculations are off by 20% or more

I would test fuel pressure


For vacuum leaks, warm up engine and let it idle
Then unplug the 2 wire connector on the IAC Valve
IAC Valve will close and RPMs should drop to 500 or engine may even stall, either is GOOD, it means no vacuum leaks
If idle doesn't drop then you have a leak

Your spark timing seem wrong, are you using a timing light or a OBD reader?
Warm idle should be about 10deg BTDC on reader, that 5-7BTDC could cause the pinging
But its not adjustable

 

Thank you for the reply! I will check fuel pressure and vacuum, and let you know the results.

 

I checked vacuum, (disconnected the IAC while it was warmed up and idling - RPM dropped a couple hundred but continued to run fine). I checked fuel pressure and had zero on the rail when the gauge was connected, about 15 to 20 psi with the key in the run position, and 40 psi when started and running in high idle. Didn't check it warmed up, but I know its throwing the PO171 during cold start / warm-up. When I shut the engine off, the fuel pressure quickly went down to zero, (within a few seconds). I'm assuming I need a fuel pump assembly, as it seems that the check valve isn't working and the pressure is lower than 55.

Your thoughts?

 

Yes, fuel pressure is low which is causing the Lean code during cold start, computer is the Choke Mode so is running the engine richer than after it warms up so needs spec'ed fuel pressure, which is, as you said, 55psi

There are two ways to lose fuel pressure with engine off, well 3 if you count fuel injector stuck open but that would cause many other issues, lol.

The fuel pump does have a check valve built-in to prevent back flow into the gas tank when pump is off
But there is also a fuel pressure regulator(FPR) on the top of the gas tank connected to the RETURN fuel line on the fuel filter
1999 and up Rangers have a 3 hose fuel filter, an IN and OUT like all fuel filters, but also a Return fuel line that goes back to the gas tank
It connects to the FPR on the top of the gas tank, its set for 60-65psi, so as pressure builds in the filter, say at idle, the pressure is relieved by the FPR letting fuel flow back into the gas tank from the filter

Never heard of these failing, and your problem is 99% likely to be the fuel pump, but not just its check valve, its only capable of 40psi when it is running
If the Return hose is nylon then it may be possible to GENTLY clamp it closed and then start the engine and see what fuel pressure is at, if it gets up to 55-60 then FPR may be the problem

 

Thanks Ron, I appreciate all the info. My fuel filter only has two connections, not sure where the regulator or return would be.

 

1998 and up Rangers got the 55psi system, prior to that it was 30-40psi

Maybe the Mazda's didn't have that in 1999, and 40 psi is CORRECT fuel pressure, but dropping to 0psi after shut off is not correct

 

Just wanted to provide an update to this issue, (now resolved with the exception of the P0171), and try to zero in on what's causing the knocking at startup ANY WEATHER and P0171 with check engine light ONLY IN COLD WEATHER...

So, reading back through this thread after several months is sort of funny. I've replaced everything expect the catalytic converter return spring, the muffler bearings and the turn signal blinker fluid lol.

Seriously though I'm almost turning 230,000 k on the odometer and this little 2.5 just keeps humming along and averages over 20 mpg consistently.

The issue with the surging was in the fuel system. I had replaced the fuel injectors because they were the next weakest link and it ran better, but it wasn't long afterwards the fuel pump eventually quit completely. Amazingly I'd already bought the pump, and I was at work, (body shop). Changed out the fuel pump and Viola! The engine ran massively better at that point, with quick starts and way more power when driving, without any surging starting or driving.

Unfortunately, there's still knocking at startup whenever the engine is cold and then it goes away and doesn't come back until the next cold start up. High octane fuel helps keep the knocking to a minimum. The other thing that's driving me nuts is that the p0171 lean code and check engine MIL come on when its below about 30 degrees F. During the warm weather it'll not pop up the lean code.

Any ideas?

Thanks!

 

ECT sensor
It sets the CHOKE on cold starts

If possible buy a Bluetooth OBD reader, under $40, it uses your smart phone(or tablet) as the display, get extension cable with it, under $10
This works on ANY vehicle sold in the US or Canada after 1995, not a Ford thing, so if you plan on driving more than a few years its well worth the investment

You can use this to see Live Data with key on, so you can see what computer sees, check what the ECT temp is before starting, should be 30deg if thats the outside temp
And if ECT temp is showing faster warm up than actual temp then fuel mix would be Lean, and knocking can occur
Also IAT(intake air temp) sensor, colder air is denser than warmer air, so IAT sensor reporting 60deg air, when its actually 30deg would cause the same Lean condition
New sensors are not "working sensors", they are just NEVER TESTED sensors, like all new parts now-a-days

The extension cable is used on some models that have OBD plugin behind a panel thats too narrow for the Bluetooth reader to fit into

 

Thank you for that info! Come to think of it, I'mm 99.9% sure the ECT and IAT sensors were among the first things I've tried, (right after flushing $$$ down the toilet with countless bottles of fuel system cleaner, etc).

It could be as you mentioned, that the new parts are just new, not necessarily working and new, so I will check them. I've got a nice MAC Tools Scout Scanner at work I can check it over with to see what they are doing.

One last thought... I had bought this truck from my older brother, who had it since it was about a year old. The good: He always maintained it, including having the timing chain/belt? replaced at some point. The bad: He always uses the cheapest backyard mechanics he can find. Honestly, as an old gear head with most of my engine prowess pre-90's, I'd think that it acts like the timing is too far advanced. Somewhere I thought I'd read the Ford 2.5 L doesn't have a knock sensor, (STUPID), so maybe the timing is off a tooth? The higher the octane I put in it, the better it is... But if I try running it with anything less than Premium 93 octane, once it starts, the knocking goes away and the engine warms up, it'll sound like a popcorn popper on crack when I'm hammered down mid throttle or beyond and accelerating. The higher octane gas all but alleviates the pinging, (I can still hear it a little). It does it much worse in colder temps after the fuel stations switch their blends. In the Summer it barely does it at all.

I guess what I'm wondering is if possibly the lean issue is not just a cold start issue, but maybe a constant problem with varying levels of severity. It just astounds me that this small but mighty cast iron engine STILL starts and runs without fail, has plenty of power and gets over 20 mpg average. At one point goofing around I fired up my drag race app and was making some runs down country roads and it gets to 60 in about 8 seconds and does the 1/4 in under 20 seconds... I'm guessing that's not bad for a 4 cyl truck.

Thanks again!!!

 

The crank sensor and its tone wheel on the crank balancer sets spark timing

If timing belt was off by a tooth you would get lower compression and a loss of power, but spark would still happen at TDC
So its not like a distributor where spark is timed by the cam shaft

Now the cranks tone wheel is a Press Fit on the crank pulley/balancer
So it is possible it shifted slightly, which could delay spark, picture of pulley and tone wheel here: https://i.ebayimg.com/images/g/21MAA...DNm/s-l400.jpg
The crank sensor "reads" the teeth on the tone wheel as the spin past it, the gap(missing tooth), tells computer when #1(and #4) are at TDC

With the MAC Tools Scout Scanner I would have a look at spark timing, should be about 10deg BTDC at idle, and of course go up with RPM
When you press gas pedal down hard, timing should Retard a bit because the Richer mix burns faster, i.e. if it was 25 BTDC, it should drop to 15 BTDC and then start moving back up as computer restores normal mix ratio

Not sure of exact 2.5l timing parameters but there should be some info online