Misfiring at low rpms, what could it be?

About two months ago I had a problem with my truck misfiring at low rpm, most prominently at 1500-2000 rpm while lightly accelerating from about 65 mph on the freeway. I changed the spark plugs, the fuel filter, and the charge motion control valve.

The problem disappeared for a few weeks. However a few weeks ago I noticed that my trucks rpms surge around 45 mph just cruising with extremely light gas pedal load. Then it expanded to 45-55 mph. Then it expanded to whenever my rpms were below 1500. Then it expanded to 2000 rpms or less when going up a hill. So basically now my truck surges between 1200 and 2000 rpms, most often in the lower range though but it is starting to happen more often in the higher of that range.

So basically my problems are coming back, again... And I have only put around 3,000 miles since I had everything replaced!

I am starting to wonder about this truck, I have put $2,500 into it and it still doesn't sound too healthy. Does anyone else hear a light piston slap when driving really slow, like in a parking lot? How about a transmission that screeches sometimes when accelerating quickly onto the freeway, particularly around 45 mph, but shifts fine besides the occasional screech?

Here are my questions

-Would you keep this truck?
-Any ideas what the misfiring is?

Oh and it's a 2006 XLT V8 with 85,000 miles.

Could be a problem with the coil(s). Try getting the engine codes scanned. It's possible to have problems that will thru "Pending Codes". These are issues that the computer keeps track of, but aren't yet severe enough to throw the engine warning light.

It is your TPS most likely. I will be surprised if it isn't.

thebrakeman said:

Could be a problem with the coil(s). Try getting the engine codes scanned. It's possible to have problems that will thru "Pending Codes". These are issues that the computer keeps track of, but aren't yet severe enough to throw the engine warning light.

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Will do, my mechanic is on a week long vacation so I will have to wait for him to get back. I hope it has a pending code, otherwise I may just end up shooting in the dark trying to fix this thing.

1992fordgreen said:

It is your TPS most likely. I will be surprised if it isn't.

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Is that a common problem on these trucks? Anything else it could be?

Trying to gather as much information as possible before I decide on fixing something. The weird part is it seems like it is kind of misfiring at around 65 mph but there is no surging in the RPMS. However when going around 35-45 you can watch the rpms surge every 3-5 seconds, especially when going uphill and not accelerating hard enough for the transmission to shift. Is all this consistent with the throttle positioning sensor not working?

Just wanted to add that I looked up TPS symptoms and my truck doesn't hesitate at all in acceleration. At least I don't notice any hesitation. It just likes to surge during low rpms. I am pretty sure it doesn't surge at all during idle or anything above 2k rpms.

How about the mass airflow sensors? Or maybe a vacuum leak? Any of this sound right?

Ford Explorer9 said:

Just wanted to add that I looked up TPS symptoms and my truck doesn't hesitate at all in acceleration. At least I don't notice any hesitation. It just likes to surge during low rpms. I am pretty sure it doesn't surge at all during idle or anything above 2k rpms.

How about the mass airflow sensors? Or maybe a vacuum leak? Any of this sound right?

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Sounds more like the throttle position sensor than the other possibilities you mentioned. Not all symptoms are evident all the time, but speed variation is often due to the sensor giving false info to the PCM and the PCM compensating with the throttle opening.

I have had the shudder and engine speed change at low and steady rpm. Ford replaced the electronic throttle sensor after my car set a code and dropped a gear at 60 mph. All has been well since the sensor was replaced.

Good luck.

Town said:

Sounds more like the throttle position sensor than the other possibilities you mentioned. Not all symptoms are evident all the time, but speed variation is often due to the sensor giving false info to the PCM and the PCM compensating with the throttle opening.

I have had the shudder and engine speed change at low and steady rpm. Ford replaced the electronic throttle sensor after my car set a code and dropped a gear at 60 mph. All has been well since the sensor was replaced.

Good luck.

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When you say engine speed change do you mean RPM or mph? I am pretty sure the rpm just surges on the truck but my speed remains relatively constant, although I could be wrong.

Ford Explorer9 said:

When you say engine speed change do you mean RPM or mph? I am pretty sure the rpm just surges on the truck but my speed remains relatively constant, although I could be wrong.

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Sorry I was not clear, I'll try again. My car was travelling at a steady speed and the rpm was also steady, when I could feel a shuddering and the tach showed the rpm changing. It did not last long and resembled torque converter shudder or overdrive shudder. It is different to engine misfire since it is slower and with larger amplitude. This happened a few times separated by several weeks. Several months later while travelling at 62mph in cruise my car slowed dramatically, dropped down a gear, and the wrench appeared in the message center window. I limped the car home and it did not feel right at all. The DTC code was P0610. The next day the car felt fine when I took it to the Ford dealer. The Ford dealer pulled P061B with their scanner and the diagnostics are horrendous and vary by engine and model (Lincoln, Ford, etc). The problem could not be diagnosed by the tests and the throttle position sensor was changed (as a guess by the tech for goodwill under warranty) and the problems have never returned.

On the older vehicles with a throttle cable and a throttle position sensor, the sensor resistance medium wears and you get hesitancy (often coming off idle) and a vehicle speed variation (speed goes up and down rythmically) in cruise control when the worn throttle position sensor gives incorrect readings to the ECM.

Good luck.

I had the same problem. Replacing the plugs and coil fixed it.

Okay so I got my truck back from my mechanic and I am just going to quote his report word for word.

"Engine Surge at 35-45 mph at warm operating temperatures. Check and advise. Torque converter clutch surge. Torque converter clutch (TCC) slip speeds vary from 0 to 150 rpm. Throttle position sensor (TPS) voltage stays steady. Commanded TCC to full lock up and TCC slip speed did not change. Inspected TPS wiring for damage and/or burnt. No damage found on TPS wiring. TCC worn internal of torque converter.

Recommend Transmission removal, replace Torque converter and TCC solenoid."

So basically he says that I need a torque converter rebuild and it can be done for $1,400. My question is does this sound right? I really really don't want to rebuild my torque converter if it isn't a guarantee.

Also does $2,000 for a full transmission rebuild sound reasonable? I figured for $600 more it would be a good price, different mechanic though, a family friend.

$ 2000 for a rebulid on 6r60 with a torque converter sounds very good to me.

I would have expected a TCC problem of clutch slip to occur at all speeds, not just at 35-45 mph.

Do you plan on getting a couple more diagnosis?

Good luck.

Yeah I am confused as to why it only happens at 35-45. Not only that but why does it only happen when warm also? I have also noticed that my mpg has gone down a bit since this started happening. Not a huge amount but about 1-2 mpg, and I am not talking about per fillup, I mean when doing 70 mph on the freeway on a flat road it has gone from 21-22 mpg to 18-19 mpg on the freeway.

I was just hoping that the diagnosis made sense to someone, because I don't want to drop $2,000 on something that might fix it...

And I will be getting one more diagnosis but it will be by the people who are going to try to rebuild my transmission.

Ford Explorer9 said:

Yeah I am confused as to why it only happens at 35-45. Not only that but why does it only happen when warm also? I have also noticed that my mpg has gone down a bit since this started happening. Not a huge amount but about 1-2 mpg, and I am not talking about per fillup, I mean when doing 70 mph on the freeway on a flat road it has gone from 21-22 mpg to 18-19 mpg on the freeway.

I was just hoping that the diagnosis made sense to someone, because I don't want to drop $2,000 on something that might fix it...

And I will be getting one more diagnosis but it will be by the people who are going to try to rebuild my transmission.

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The PCM usually does not allow TCC lockup until the engine/transmission temperatures are warm. Hence all tests start with normal operating temperatures. Also, a loss of mpg at 70 mph would be consistant with the TCC not being locked, but the road elevation and wind conditions, etc can play a role too. Here is what the Ford Service manual has to say about trans temperature:

Transmission Fluid Temperature (TFT) Sensor

1. The transmission fluid temperature (TFT) sensor is located on the TCM. The TCM uses the sensor input to activate various shift strategies. The sensor is in the form of a temperature-dependent resistor.

2. The resistance value of the TFT sensor varies with temperature change. The TCM monitors the voltage across the TFT to determine the temperature of the transmission fluid.

3. The TCM uses this initial signal to determine whether a cold start shift schedule is necessary. The TCM also inhibits torque converter clutch operation at low transmission fluid temperatures.


The normal operation of the torque converter (without the clutch engaged) is to increase the rpm a few hundred rpm with a slight increase in throttle opening. The engine is quite responsive in changing rpm. With the torque converter clutch engaged the rpm climbs slowly and steadily with slight increases in throttle opening and so the engine speed is not nearly so responsive. So you can use a cold engine/trans to view engine rpm with the TCC unlocked and compare to operation at normal operating temperatures.

When driving my son's 2007 V8 Explorer the rpm at 62 mph (100 kph) is about 1,750 which is the same as my car. I don't know about 70 mph but maybe about 2000/2200. If the TCC is not engaged then rpm should be noticeably higher.

The Ford service manual test for TCC lockup is:

Torque Converter Operation Test

The Torque Converter Operation Test verifies that the torque converter clutch (TCC) control system and the torque converter are operating correctly.

1. Carry out Quick Test with scan tool. Refer to the Powertrain Control/Emissions Diagnosis (PC/ED) manual. Check for DTCs.

2. Connect a scan tool to the vehicle.

3. Bring the engine to normal operating temperature by driving the vehicle at highway speeds for approximately 15 minutes in D position.

4. After normal operating temperature is reached, maintain a constant speed of about 56 km/h (35 mph) in 5th gear for 10 seconds.

5. Release the accelerator pedal and monitor the scan tool TP voltage to approximately 1.25 volts or 30% throttle.

6. Monitor the TCC and engine speed PID. The TCC should release and engine rpms should increase before the 5-4 shift occurs.

The test for the V6 is much simpler, since at steady 50 mph just tap the brake pedal (left foot) while maintaining throttle position and watch the engine rpm rise noticeably as the PCM disengages the TCC. So the V8 tends to hold onto the TCC lockup longer and only releases to shift gears.

Good luck.

Finally got it fixed! Took my truck in and had them replace the torque converter, problem solved!

Just for future reference my symptoms were:

30-50 mph with light acceleration my rpms would jump up about 150 rpm every few seconds. Felt similar to a misfire. Only happened at rpms lower than 2000.

It cost $1,360 for the dealer to replace it. At least it works now though!