Code P0300 Random/Multiple Misfire and loss of power under load

[RKCRLR]

2000 K3500 7.4L

I've been having this problem for years now.  When the truck is under a load for a while (like when hauling my cabover camper up a grade) the check engine light will come on and the truck will lose power.  It will also not downshift as easily as it normally does.  I can sometimes feel it happen before the light comes on, almost like a switch is thrown.  Sometimes I can let off the throttle and stomp on the pedal to get some power back for a while.  Initially I would pull the codes and the only thing present was P0300 Random/Multiple Cylinder Misfire Detected.  If I clear the code it will initially have more power until I go up a grade again.  The temperature gage is usually right in the middle to a little above middle and no signs of overheating.  I had also checked my fuel pressure (but not while driving) and it was within specification.  

 

Now the P0300 code is accompanied with pending low voltage codes for all 4 O2 sensors (P0131, P0137, P0151, and P0157).  

 

Things I have replaced:

All 4 O2 sensors

Distributor, (including cap and rotor)

Spark plug wires

Spark plugs

Engine

Fuel filter (about 8K miles ago)

 

I haven't replaced the ignition coil.  I know the fuel pressure regulator leaks because the engine will crank for a while before it starts after it has sat for a bit but not cooled down.  It starts fine when cold and when hot if restarted soon after it is turned off.  I guess the next things I'm considering is a bad MAF.  I considered a plugged exhaust but that doesn't make much sense since it will initially run good and then abruptly loose power although it does "feel" like restricted exhaust or lean mixture when it happens.  

 

I have a OBDLink MX+, Torque Pro, and OBDLink.  What should I monitor during driving to help narrow this down (I'm getting tired of troubleshooting by replacing parts)?  Anything I can test for?  

 

Edit

I took the MAF off and didn't see anything obvious.  No debris and not oily.  I also did the test where you check to see if the flow rate doubles each time the RPM doubles and it did (I didn't go over 4K RPM).  

I also checked the throttle position with the engine off but ignition on.  0% at no throttle and 100% when floored.  

There is an option for fuel pressure in Torque Pro (which interestingly disappears once connected to the OBD2 reader) but it always stays at zero.  But fuel flow rate is available.  

From what I have researched the low O2 voltage indicates a lean (vs rich) mixture.  I guess the next step is to see if I can reproduce the problem while monitoring my Long Term Fuel Trims to see what they do.  I'd drive around with my fuel pressure gauge taped to the windshield but my hose isn't long enough.  

Edited August 8, 2024 by RKCRLR

[richard wysong]

Try checking the fuel volume, may be a clogged sock in the tank. A quick test can be done with a fuel pressure gauge with a dump valve on it, the engine should stay running with little change with the dump valve wide open or you can look up the specs and get a graduated container and a stop watch and do it by the book. Watching the fuel pressure while the problem occurs may help and I'm sure the bad FPR does not help 

Edited August 8, 2024 by richard wysong

[RKCRLR]

30 minutes ago, richard wysong said:

Try checking the fuel volume, may be a clogged sock in the tank. A quick test can be done with a fuel pressure gauge with a dump valve on it, the engine should stay running with little change with the dump valve wide open or you can look up the specs and get a graduated container and a stop watch and do it by the book. Watching the fuel pressure while the problem occurs may help and I'm sure the bad FPR does not help 

By dump valve I assume you mean the push button pressure relief valve on my fuel gauge.  It has a hose on the valve so I could route it to a container and press the button while the engine is running.  Any idea what kind of flow rate I should expect through that valve with the engine idling?  

 

Thanks

 

Edit: I found a rating for 65 GPH (about 1 GPM) online.  Does that seem reasonable?  It didn't state the pressure at that flow rate.  

Edited August 8, 2024 by RKCRLR

[richard wysong]

That is probably correct, all vehicles have a pressure and volume spec listed in the service manual. Doing the dump valve test the pump should be able to maintain enough pressure and volume to stay running, A lot simpler than the actual volume test. Doing the volume test there would not be fuel pressure as you are checking the wide open flow of the pump

Edited August 8, 2024 by richard wysong

[RKCRLR]

Here's my results:

 

Fuel pump running (ignition on) and engine off: 58 psi for a couple of seconds and then drops to 50 psi when the pump stops.  The pressure slowly increases to about 55 psi over about 5 minutes (nothing running).  I'm assuming this is because of heat soak.  

 

Fuel pump running and engine idling: 50 psi.  When I press the relief button on the gauge the dial drops to zero but no change in engine idle.  I suspect there isn't much flow rate through the gauge connection on the fuel port.  I only got about a cup in about 30 seconds.  

 

Also, I have an electronic version of the service manual for my truck and I couldn't find a fuel pump flow rate specification anywhere.  

 

[RKCRLR]

I got a hose extension for my fuel pressure gauge and drove around with my fuel pressure gauge taped to my windshield.  I got the truck to act up and the fuel pressure was still around 58 psi when it was losing power and the check engine light came on (P0300, P0137, and P0157).  

 

I logged fuel trim and Bank 1 O2 sensor values in Torque (I screwed up and didn't log bank 2 O2 sensors) and attached the file.  I don't know enough to know what the data should look like.  If anyone wants to take a look it would be appreciated.  

 

I've ordered an exhaust pressure gauge and will check for a clogged catalytic converter next.  

trackLog-2024-Sep-04_13-14-19.ods

[asilverblazer]

The O2 sensor codes are for the downstream sensors. Those should not factor into the PCM's fuel trim and should not be CAUSING the miss fire, they could be detecting a lean condition caused by something else and are left to interpret it as a sensor low voltage condition. This would mean that the upstream sensors are either not accurately sensing the air fuel ratio - thus the PCM doesn't enrich the mixture accordingly, the PCM may not be getting an accurate signal from the sensors via a wiring fault, or the sensors are fine, the PCM is reading them correctly but isn't able to deliver the fuel volume needed due to a mechanical fueling problem, fuel pump, filter, or injectors (maybe dirty).

 

I would watch fuel trims to see what the PCM is trying to do to correct the P0300. High fuel trims indicate its commanding a richer mixture vs. low trying to lean it out. A high fuel trim with lean O2 readings indicate an insufficient fueling problem.

[asilverblazer]

Looking at your data, the truck runs lean at full throttle. Not seeing it richen the mixture accordingly via the fuel trims though, not really studying it fully though. 

 

Based on what I see I'd be looking at fuel delivery.

[RKCRLR]

1 hour ago, asilverblazer said:

Looking at your data, the truck runs lean at full throttle. Not seeing it richen the mixture accordingly via the fuel trims though, not really studying it fully though. 

 

Based on what I see I'd be looking at fuel delivery.

That makes sense and is sort of what it feels like.  But since the fuel pressure stays around 58 psi when the problem is happening it would mean that the fuel pump, fuel filter, and regulator is doing their job and mean that it is the injectors (more than one) if it is a fuel problem, wouldn't it?  Would plugged cats cause low voltage on the O2 sensors?  

 

I could do another run monitoring bank 2 O2 sensors and whatever else you think would be helpful.  

Edited September 5, 2024 by RKCRLR

[richard wysong]

You can have good fuel pressure with insufficient volume, does your fp gauge have a dump valve on it? If so open the dump valve with the engine running, it should stay running if the volume is ok

[RKCRLR]

16 minutes ago, richard wysong said:

You can have good fuel pressure with insufficient volume, does your fp gauge have a dump valve on it? If so open the dump valve with the engine running, it should stay running if the volume is ok

Yes, the engine keeps running with dump valve open.  

 

I'd expect the fuel pressure to drop at full throttle when the problem is happening if the fuel flow volume to the injectors is insufficient.  

[asilverblazer]

19 hours ago, RKCRLR said:

That makes sense and is sort of what it feels like.  But since the fuel pressure stays around 58 psi when the problem is happening it would mean that the fuel pump, fuel filter, and regulator is doing their job and mean that it is the injectors (more than one) if it is a fuel problem, wouldn't it?  Would plugged cats cause low voltage on the O2 sensors?  

 

I could do another run monitoring bank 2 O2 sensors and whatever else you think would be helpful.  

I wouldn't focus on the downstream O2 sensors reading low voltage; yet... Log should include bank 1 sensor 1 and bank 2 sensor 1.

 

I suppose there could be a scenario where clogged catalytic converters are slowing the exhaust gases so much that it is altering the upstream sensors readings, and if the exhaust gas is spending to much time in the catalyst media, it could cause a low voltage reading on the downstream sensors. Drop the y-pipe and see what happens?

 

Quote

Other Ways of Diagnosing Catalytic Converter Failure

 

Using a Vacuum Gauge

One quick and easy test is to connect a vacuum gauge to a manifold vacuum port (like the one feeding the brake booster). Idling on a healthy engine, the gauge should read 18-22 inches of vacuum idling in park or neutral. Wear safety glasses and remain clear of moving parts while you have an assistant raise the engine speed (still in park or neutral). If you can operate the throttle yourself from under the hood (be careful) that works too.

On a healthy engine, the vacuum should dive very briefly and then recover to a slightly higher reading than where it was at idle. If the vacuum drops with the engine speed raised and stays lower than it was with the engine idling, the catalyst is probably clogged. If raising the engine speed a little more lowers the needle even further, it’s more certain. At this point, move to the next step below.

If you have the tools to do it (this can be tough on some vehicles), One of the first things an experienced mechanic will do to trace a clogged converter is they will temporarily remove the oxygen sensor or unbolt the exhaust downpipe. If engine performance improves with the sensor or the exhaust removed, chances are high that the catalytic converter is the component that’s causing the problem. Of course, the engine is extremely noisy when the sensor is out or the exhaust is removed and if you drive the vehicle with the O2 sensor removed or the exhaust disconnected, the heat can be intense enough to cause damage to nearby components, so don’t run the engine too long or too much with the O2 removed or the exhaust disconnected.

You may also check the structure of the converter for any impact damage caused by running over road debris, but any road debris that has damaged the converter usually causes other noticeable damage.

 

Testing for Backpressure

There’s another way of confirming your suspicion—by measuring the backpressure. To do this, you’re going to need a low pressure gauge with a scale that reads up to 15 PSI. You may also opt for a basic backpressure test kit (available from online suppliers for $20-$30). These kits typically require removal of the O2 sensor.

If you want a more accurate reading, you can use a digital manometer ($40-$50 online), or a pressure gauge with a variety of units of measurement. Any of these require some kind of connection to the exhaust in front of the converter being tested.

If your engine has a secondary air injection system, you can disconnect the check valve and install your pressure gauge. You may get a more accurate result if you connect the check valve to the exhaust system before the converter. But note that connecting to the check valve (which is usually a ⅝ hose connection) would require building some type of adapter).

You could also drill a hole along the exhaust pipe (just ahead of the catalytic converter) where you can attach the pressure gauge. This is your best option if you have an older car and are worried about damaging the O2 sensor. If you drill a ⅛ hole you can simply hold a square cut hose up to the hole while an assistant increases engine speed. After the test you can install a self-tapping screw or, better yet, a blind pop rivet in the ⅛ hole.

The backpressure in an idling vehicle can vary depending on its year, make, and model. Typically, an engine at idle should have 1.5 PSI at most. Of course, some engines can go way higher, but the rule of thumb is to have 1.5 PSI and below. Less than 1 psi is optimum.

When you rev the vehicle at 2,000 RPM and it remains steady at or below 3 PSI, then you likely don’t have issues in your catalytic converter. But if it fluctuates or increases beyond 3 PSI despite having a steady RPM, it’s a good indication that backpressure is building up.

Revving up the engine will increase the pressure, which is normal. However, an abnormal increase in pressure at a steady RPM can indicate possible backpressure.

 

Other Ways to Check Back Pressure

It’s worth noting that you can also check back pressure with a vacuum gauge (not a pressure gauge). Connecting a gauge to a source of manifold vacuum is much easier than connecting a gauge to the exhaust.

Additionally, it may be possible to get an idea of back pressure by monitoring a scan tool and looking at parameters, such as manifold absolute pressure (MAP) and calculated load.

 

Edited September 6, 2024 by asilverblazer

[RKCRLR]

10 hours ago, asilverblazer said:

I wouldn't focus on the downstream O2 sensors reading low voltage; yet... Log should include bank 1 sensor 1 and bank 2 sensor 1.

 

I suppose there could be a scenario where clogged catalytic converters are slowing the exhaust gases so much that it is altering the upstream sensors readings, and if the exhaust gas is spending to much time in the catalyst media, it could cause a low voltage reading on the downstream sensors. Drop the y-pipe and see what happens?

 

 

I can monitor vacuum using Torque Pro.  I checked at an idle and reving the engine and it didn't indicate a blocked exhaust.  My brother-in-law (who is a mechanic and services rafting vehicles) thinks it is a clogged cat (he says he sees a lot of these) and suggested removing the upstream O2 sensors and driving around.  I didn't like where the exhaust could impinge when doing this and decided to order an exhaust pressure gauge.  

 

I might try driving around with the upstream O2 sensors removed or I might try logging the intake manifold vacuum and the bank 2 sensors if the exhaust pressure gauge doesn't work out.  

 

Thanks for your input!

Edited September 7, 2024 by RKCRLR

[RKCRLR]

So I think I can rule out clogged cats.  I got a cheap chinese backpressure gauge.  The only time the needle would move off the peg is when I'd rev the engine and then it only bounced and went back on the peg.  It stayed on the peg at idle, steady 4K RPM, and floored in gear at 2K RPM.  While the gauge isn't very accurate I checked it against a couple of tire gages.  It appears the first line is about 3 psi and it never got there (other then bouncing a little when revving the engine).  So the exhaust pressure doesn't ever seem to exceed 3 psi.  

 

So any recommendations on what to monitor if I take it for another drive?  O2 sensors on both banks, manifold pressure, fuel trims?  Anything else?  

 

Edit: I guess it would be good to monitor timing advance to see if it is pulling timing.  

Edited September 30, 2024 by RKCRLR

[CamGTP]

Just curious, what brand o2 sensors did you use?