3800 engine runs rough at cold start up

It sounds like you've already taken several steps to address the rough running issue on your 1996 Buick Regal with a rebuilt 3800 engine. Since you've replaced several sensors and components related to engine performance, there could be a few more possibilities to consider:

Vacuum Leaks: Even if you've replaced the sensors, there could still be vacuum leaks in the intake manifold or other areas of the engine. Vacuum leaks can cause rough idling, especially when the engine is cold.

Idle Air Control (IAC) Valve: The IAC valve controls the engine's idle speed. If it's malfunctioning or dirty, it can lead to rough idling during cold starts. Cleaning or replacing the IAC valve might help.

Fuel Injectors: Dirty or clogged fuel injectors can disrupt the fuel-air mixture, causing rough running. Consider having the fuel injectors professionally cleaned or replaced if necessary.

EGR Valve: A malfunctioning Exhaust Gas Recirculation (EGR) valve can cause rough idling. Make sure the EGR valve is functioning properly and not sticking open or closed.

Air Intake and Throttle Body: Check the air intake system and throttle body for dirt, carbon buildup, or obstructions. A dirty throttle body can affect airflow and lead to rough idling.

Used injectors but cleaned. Plugs and wires are new and rechecked. The car did sit for 11 months with a full tank of fuel treated with sea foam. I have a scan tool and had no output from the sensors I replaced. The car starts and runs perfectly every time when it gets to 150 deg. F vida the scan tool!

Monitor the coolant sensor when cold

Roy

70 Deg. this morning and out side temp. was 57.2 so I replaced it today with no change

Air intake temp. was 47 Deg.

air intake will not effect it. it is the coolant sensor, the temp of the coolant. it gives a richer mixture when cold. it may be running lean

Roy

that was what I thought and why I replaced it a second time day the first was when I rebuilt the engine! I still have the original should I try that?

the temp sensor you said, was that the sensor by the thermostat housing or on the back of the head

Roy

Thermostat Housing

what was the reading on the scan tool when cold??

Roy

70

if that is degrees, that sounds right.

what about the mass air flow??

Roy

I'm not getting a code!

you dont need a code for it to perform poorly. besides, a code never identifies a bad part just a failed system

Roy

The temp. sensor I replaced has 3 wires and the book said in that case it will have two and one of them will have one wire! Does this sound right I'm not finding it! The gage in the car does read high the fan comes on at 230 Degrees and the gage it just under the red almost on it!

what did your scanner say when the temp was up. did the reading on the scanner match the gauge??

Roy

Reading on the scanner was 230 the dealer told me that was normal but the gage had never gotten that high before the rebuild!

The Mass Air Flow Sensor is putting out 2.687 volts at 112 degrees (engine temp) and 740 RPM and I raised the RPM to 3000 and no change.

the voltage should have changed in proportion to the engine speed. if it did not, then you may have a failed mass air flow.

Roy

Ok the Voltage never changed I will have to get a new one thanks!

The pounds per inch changes on the scanner at higher RPM

as you increase rpm, the voltage should respond imeediately.

Roy

I took out the MAF and cleaned it the car started cold and ran better but not right Idle was all over and the voltage on the MAF sensor was at 2.60 and went up to 2.80 at 3000RPM

I did find some small cracks in the top of the sensor looks like I must have hit it when I rebuilt the engine!

try tapping on t when it is running and see if the pitch of the motor changes

Roy

I cant here it but I can see the RPM on the scanner change

i think you need to replace the mass air flow sensor

Roy

Ok Thanks I will let you know how it comes out in a week or two

good luck

Roy

I haven't got the new sensor yet I got one off ebay and it was damaged so I'm waiting for a replacement. My dad looked at the car on the scanner and said the TPS was unstable and thinks it maybe a ground problem with the PCM do you know where the ground Is I cleaned the ones at the bell housing? I do see one looks like an 18-16 Ga. wire by the battery tray that looks suspect but the bolt is so badly rusted I haven't had the time to get it off yet! could this be the problem I forgot that the speedometer is off by 3 MPH could that be related to the ground? and can you tell me where the speed sensor is? Thanks

Ted
P.S. I'm still planing to replace the Mass Air Flow Sensor because its damaged

I replaced the TPS and its fine now. I also replaced the MAF sensor and that is unstable and bounces up and down. I had the Idle speed set at 600 R.P.M.'s and that sets a code for random misfires I turned it up to 750 and no comes but if I rev it up and let go of the throttle is drops to 450 and picks back up to 750 but sometimes it will stall. Do you have any Ideas?

Ted