Hi Jeff, CM here. To answer your question directly as to what the voltage reading should be- for a TPS, (if not modified) at idle: 0.68 volts give or take a few. At WOT, I've never seen any of them, even OEM's reach 5 volts. It was assumed that the range was 1 to 5 volts but, I've never seen it meet those specs.

Now, let me explain a bit about the EFI system and how it works so you can trouble shoot better as to what it may be.

The problem here is, there's more than one sensor and, if any one were to short, go out or, fault in anyway, it has a systemic affect on the rest of the EFI system thus, throwing a lot of other readings out of whack.

I'm going to go over a few of the sensors in this system as, these are usually the ones that go out and cause what you're describing.

There's two master sensors that are responsible for time keeping and they are: CKPS and CPS. One is responsible for time keeping on the Crank (CKPS) and the other is used at the CAM (CPS) level for reference signal at input to the PCM.

Now, the third Master sensor is the MAP sensor. This Manifold Absolute Pressure sensor is responsible for providing input to the PCM in regards to the VAC level inside the intake AND, it also plays a huge role in fuel mapping and transmission shift points and timing among other functions. But, as for bogging, those are the functions that have an affect on HOW the engine responds to throttle position. A FUEL MAP table is updated by the results of the MAP sensor and, depending on the conditions, the A/F ratio either richens or leans out respective to operating conditions. Usually, bogging is due to too much fuel- rich mix.

That being said, when you TROMP on the TPS, this sends a higher voltage at INPUT of the PCM. The PCM in turn USES the DATA from the FUEL MAP TABLE and, it uses those numbers to adjust the FUEL AMOUNT but, also, the SPARK TIMING / ADVANCE. More fuel is pumped and for longer dwell time at the inejetor level.

If this MAP sensor is faulting OR, there's a VAC LEAK, the tables stored in the PCM will not match what's really going on inside the engine thus, throwing the FUEL RATIO way out of whack as well as the TIMING thus, it will bog like a dog.

I recommend to check for vac leaks as well as having that item put on a scanner and plot out what it's doing. A dealer will be able to scan the whole thing in minutes for you for about 100.00 OR, if you've got a decent scan tool (300.00), you can plug it into the OBD port under the dash on the drivers side and go through each sensor readings.

If the TPS were faulting, USUALLY: IT JERKS, or, IT LOPES during use thus you feel a constant oscillation or surging movement as it speeds and slows during normal driving conditions. If it's got a dead spot in it's range of travel, a drop out will be seen on the graph while under use. Or, you will FEEL it cut out.

If it were ignition related thus, bad plugs, wires, cap and rotor, only one (or more) cylinders would be affected. So, for now, you can factor that out completely.

BUT, there IS a component in that system (IGNITION SYSTEM) that WILL create a bogging issue if and when it goes out and that is, THE COIL.

IF the coil is not putting out full peak voltage or, is not turning on and off in time, the voltage may be insufficient to ignite the mixture and / or, the timing may be off at higher rpm speed thus, loss of power and bog-down.

On that same failure mode, it is also quite possible to have what is called a fault in the COIL DRIVER CKT at the PCM level. You see, the PCM is responsible for switching GROUND of the coil ON/OFF in time with the fuel system. There may a voltage drop across the switch. If this goes out, a new PCM is the solution.

There's also another parameter called FUEL SYNC. This is part of the programming associated with the FUEL DELIVERY SYSTEM that tells the PCM when (timing) and for how long (duration) to fire the FUEL INJECTORS.

The master sensor that I mentioned before (CPS) for the CAM, is a Hall affect sensor that resides in the Distributor housing and, as it goes around, it pulses a low voltage signal to input of the PCM for reference. This is how the PCM can time the fuel injector sequence over the rpm range.

Now, it is all too often that the PCM LOSES, sync and, a simple test to see if that's happening is to simply disconnect the NEGATIVE BAT terminal for apx 1-3 minutes and reattach after that time has been met.

Test the truck and, if it is resolved or at least, not as bad- you've got a potentially unstable FUEL SYNC issue to which, again, the dealer or, you can reset this value if you've got a GOOD diagnostic scanning tool that has this feature. These are usually more expensive and, it only costs 100.00 at the dealer to have this checked and reset if need be.

It also may be as simple as a faulting O2 sensor that is making the vehicle run too rich. It's input voltage may be far enough off from real world measurments that, the PCM interprets a lean running vehicle and, when tromping on it, the mix is just way too rich to burn thus the bogging.

Those are some of the reasons for bogging when looking at it from a electronic control systems perspective.

The problem when replacing sensors without testing is the cost for, the MAP sensor alone can fetch up to 180.00 just for that one sensor. Oh, speaking of sensors, before I forget, do yourself a favor and don't go with after market / 3rd party sensors. These are not high qual, fully tested / screened parts and, most of them are actually rejects that don't make it to the dealers and are bought off by brokers who in turn, turn around and sell them to part stores across the country. They are not gross failures mind you but, they may perform sporadically and intermittently such as a voltage drop out during half throttle position on a faulty TPS.

Now, while that seems like a daunting list and, it gets even worse when looking at ALL the sensors, it is quite a possibility that it is CAT related as was already mentioned above. If so, there's a heating up of the engine associated with a clogged CAT. If it's restricted bad enough, the exhaust can't escape thus, it backs up into the rear of the engine first and begins to overheat the back two CLYs No 7, & No 8. Keep an eye on the temp gauge. FYI

CM

 

Thank you for the in depth write up, good info!

1. Engine is not heating up, running normal temps.

2. Checked plugs yesterday and no signs of being rich at all. Plugs were nice and clean.

Will try the neg battery disconnect today. It is very cold here today but I can tough it out for this test.

Thank you again for your time to type this all out!

Thanks, Jeff

 

No problem Jeff. Typing it all out is not an issue. Even though the plugs look good, it still may have a rich fuel mixture at certain TP positions. When running back at normal speeds, all that burns off and, by the time you pull them all out to inspect them, they look fine thus, making it appear as if there's no issue with the fuel mixture.

Here's another indicator of a seriously clogged CAT; BACKFIRING or popping through valves due to back pressure. If it's only partially restricted, it will slow and bog, no popping or backfire will be witnessed and, the engine should run at normal temps as well.

Usually, when loading the engine such as going uphill, it will exhibit signs of bogging down while under load. It's still a good idea to pull the front O2 sensor out completely yet leave it connected and hanging by a wire tie and test it for SHORT duration to see if the problem remains constant. If so, you most likely have either a timing or A/F ratio issue.

Do you happen to know what the fuel pressure is while under throttling to the bog point? See, you may have adequate pressure at idle and at around 1500-2500 rpm but, when laying into it, the FUEL PUMP may not be flowing enough fuel at higher RPM loads thus, a drop in pressure would be seen on the gauge under load conditions.

I would also verify fuel pressure under use. You can simply attach a fuel rated pressure gauge onto the Schrader Valve at the top of the fuel rail. Remove the cap, hold a rag over the port, depress using a screwdriver to release pressure and then attach the gauge to the port. Get an extended hose and run it up through the firewall opening (where harnesses run through a rubber grommeted opening) so you can monitor it while driving. You'll want to run the line through the firewall first and then attach it to the port.

If you can't find a long enough hose, you can attempt to rev the RPMs via the throttle shaft and note a drop or not on the fuel gauge at higher rpms. The spec is 49 psi ± 5.

If at any time, it drops below the MIN psi, the fuel pump is faulting. NOTE: when cold, PSI rating may exceed MAX rating. This is fine. When truck heats up, that same reading will drop to normal operating temperature.

I'm not saying you've got a fuel pressure issue but, rather on how to TEST for a faulting pump while under loading conditions.

Let me know what you find out.

Regards,

CM

 

I went out and unhooked the Neg cable for at least 10 mins, came in and warmed up. Went for a drive and no change.

I checked the fuel pressure at the connection below the drivers side floor as I don't have a schrader valve on the fuel rail. My fuel gauge does not have an adapter for inline hookup. I will poke around and see what I can come up with for an adapter.

Only 20° here today so I am pretty much done for now and suppose to be only around 10° for highs next few days and lows from -10° to -15°.

Thanks, Jeff

 

I hear ya on the temp outside. I miss So Cal as I can work on the truck anytime of year without an issue. Out here on the east coast, it sure is a pain to work outside. You would have to create an line gauge that goes between the supply line and the rails and, to do that, the lines would need to be changed out or, at least the fittings. Now, if you've got plastic supply lines, I would rid of those and replace them with SS Flex lines if it comes down to it. Then, you'd be able to insert a gauge in series with the rails to see what's up while running. Without verifying pressure during operation, we have no way of knowing what the supply is doing and, it very well could be as simple as a insufficient fuel pressure under running condition. The lines most always will pump up to full PSI but, depending on the demand needed (flow rate) the psi can drop.

I'll check back in a few days or so.

Keep warm.

CM

 

Thanks for all the help guys! It was the cat. We are up and running again. Sorry for the long wait in replying was too busy enjoying the nice weather for a while. I always post back with the fix, just slow sometimes. Hope this helps someone in a search sometime.

Thanks again guys!!!

Jeff

 

Awesome! I need to check mine - even with all of the recent work (pretty much top end rebuild except heads) it seems to bog as well and still 10-12 mpg. All common sensors replaced except Idle Air Control - will disconnect new O2 sensor and check CAT today.

Great write up!

 

Mine did the same bogging out my air filter was nasty dirty. Cleaned it up and ran done never had the problem again

 

I disconnect O2 sensor and ran some errands. YUK - I hated the sound. I still have what I call bog down. I'm almost convinced I'm getting too much air and not enough spark after the kegger mod.