Warm engine shudders and runs rough at startup.

[luvmonterey]

Hi all. My '94 with 218k has a habit of shuddering and running really rough when I first start up a warm engine. When the engine is started cold, all is fine, but if I drive to my destination, like the hardware store, park, and come back 10-60 minutes later, it shudders and shakes until I get my foot on the gas and start driving. Once moving, all is fine, as though nothing is is/was wrong. It's a bit embarrassing with passengers and I want to fix it, but not sure where to start. I do most of my own repairs, so I have the know how, but not sure where to begin. Anyone else ever have this problem?

 

[FR-425]

There are two temp sensors on this engine. One is for the temp gauge, the other is for the computer.

That's your bad guy. Rough starts after engine is warm is the symptom. The fault is the ECT (Engine Coolant Temp) sensor.

They are both located at the Thermostat housing. The one toward the rear of the engine is the ECT. The ECT has a sharp point. Keep this in mind when at the parts store, they almost always hand you the wrong one.

 

[luvmonterey]

There are two temp sensors on this engine. One is for the temp gauge, the other is for the computer.

That's your bad guy. Rough starts after engine is warm is the symptom. The fault is the ECT (Engine Coolant Temp) sensor.

They are both located at the Thermostat housing. The one toward the rear of the engine is the ECT. The ECT has a sharp point. Keep this in mind when at the parts store, they almost always hand you the wrong one.

I will give this a try, thanks!

 

[luvmonterey]

Do I need to drain the radiator in order to replace this sensor? Thanks!

 

[FR-425]

nope

 

[luvmonterey]

nope

Both sensors appear to be the same distance from the engine. One is directly behind the thermostat housing and is smaller with a small connector on the tip. The other is to the right, and below the throttle body. It is larger and has a much larger connector. The ECT sensor is the smaller one? Thx.

 

[FR-425]

the one one the right

 

[FR-425]

Remember to disconnect the battery before you start.

 

[luvmonterey]

Remember to disconnect the battery before you start.

Yes, always disconnect!

 

[luvmonterey]

I ordered the ECT sensor from Amazon today. It was $10 cheaper than the local chain store and free shipping. I will install it next weekend. Thanks for the info!

 

[FR-425]

FYI.
The reason this problem occurs is that the PCM thinks the engine is cold and starts the engine as if it were a cold start, which runs the injectors at full rich mixture.

Basically flooding the engine.

After the normal cold start high idle time runs down the mixture leans out a bit. But not fully. It will run okay but fuel econ and emissions will suck.

Once the high idle timer finishes the IAT (Intake Air Temp) sensor provides 1/3 of the info the PCM needs to set the fuel mixture. The next 1/3 is engine temp. (ECT) and finally Oxygen sensors for the last 1/3.

Unfortunately Diagnostic code 21 (ECT) out of range does not always set. Particularly if shorted/grounded. Which happens to be the common fault state. (Thanks Ford!)

This can also lead to a nightmare of faults due to the short causing the "reference" voltage to drop well below 5v, if you also have a weak battery or bad ground strap.

 

[luvmonterey]

FYI.
The reason this problem occurs is that the PCM thinks the engine is cold and starts the engine as if it were a cold start, which runs the injectors at full rich mixture.

Basically flooding the engine.

After the normal cold start high idle time runs down the mixture leans out a bit. But not fully. It will run okay but fuel econ and emissions will suck.

Once the high idle timer finishes the IAT (Intake Air Temp) sensor provides 1/3 of the info the PCM needs to set the fuel mixture. The next 1/3 is engine temp. (ECT) and finally Oxygen sensors for the last 1/3.

Unfortunately Diagnostic code 21 (ECT) out of range does not always set. Particularly if shorted/grounded. Which happens to be the common fault state. (Thanks Ford!)

This can also lead to a nightmare of faults due to the short causing the "reference" voltage to drop well below 5v, if you also have a weak battery or bad ground strap.

Thank you for the info. I'm really looking forward to having this issue resolved!

 

[MrShorty]

Now that you have already ordered the ECT, I will mention the other common cause of rough warm starts that I see: a bad Fuel Pressure Regulator. One failure mode of the FPR is to develop a leak in the diaphragm. When this leaks, fuel leaks into the intake through the vacuum line of the FPR, again, effectively flooding the engine. The engine will start rough (if it starts at all) until the excess fuel dissipates, then run fine. After enough time has passed, the excess fuel dissipates on its own and the engine will start cold just fine.

Test is easy -- run the engine for a moment, then pull the vacuum line off of the FPR. If gasoline drips out of that vacuum line, the FPR needs replaced.

 

[luvmonterey]

Now that you have already ordered the ECT, I will mention the other common cause of rough warm starts that I see: a bad Fuel Pressure Regulator. One failure mode of the FPR is to develop a leak in the diaphragm. When this leaks, fuel leaks into the intake through the vacuum line of the FPR, again, effectively flooding the engine. The engine will start rough (if it starts at all) until the excess fuel dissipates, then run fine. After enough time has passed, the excess fuel dissipates on its own and the engine will start cold just fine.

Test is easy -- run the engine for a moment, then pull the vacuum line off of the FPR. If gasoline drips out of that vacuum line, the FPR needs replaced.

Thanks for the information. I will check this out. One question though, does the test need to be run with the engine warm or cold? Thanks again!

 

[MrShorty]

It just needs to be long enough so that, if there is gasoline leaking into the intake through the vacuum line, there will be enough in that hose for you to see.

 

[luvmonterey]

It just needs to be long enough so that, if there is gasoline leaking into the intake through the vacuum line, there will be enough in that hose for you to see.

Ok. Thanks!

 

[larrydd999]

This site is amazing! I've just noticed the same problem and was wondering what it is... Damn, you guys are good!

 

[cbach1997]

Hi all. My '94 with 218k has a habit of shuddering and running really rough when I first start up a warm engine. When the engine is started cold, all is fine, but if I drive to my destination, like the hardware store, park, and come back 10-60 minutes later, it shudders and shakes until I get my foot on the gas and start driving. Once moving, all is fine, as though nothing is is/was wrong. It's a bit embarrassing with passengers and I want to fix it, but not sure where to start. I do most of my own repairs, so I have the know how, but not sure where to begin. Anyone else ever have this problem?

I have for FOREVER and have never chased down the problem. This thread has me excited. I replaced the original coil pack thinking with 280,000 miles on it it has to be toast, and surprise surprise it got less frequent but didn't disappear entirely. I will pursue this thread's recommendation next.

 

[Masterd]

I'll add to this. I've experienced the same issue and eliminated the FPR as the issue, even waiting for a long time for it to produce any signs of fuel leakage. I've always assumed it was something getting heat soaked on shutdown and then cooling off sometime after restart, like a fuel injector. In my case, being a '93 with over 300k miles, everything should be suspect.

 

[DrewSmith007]

I had a loose vacuum line that would cause the truck to run rough at idle when warm. Mine was toward the back of the engine on the driver side, just above the oil dipstick.