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2004 DTS IPCu1000 PCM p0300. Car runs fine then idles rough…

Customer Question
2004 DTS IPCu1000 PCM p0300...
2004 DTS IPCu1000 PCM p0300. Car runs fine then idles rough and accelerates very rough for about 5 min. Like it is running on 4 cylinders. Then it will clear up and be just fine for 50 to 100 miles. I replaced all plugs and TPS. Cleaned throttle body and mass air flow
Submitted: 2 years ago.Category: Cadillac
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Customer reply replied 2 years ago
I have checked for vaccum leaks by spraying throttle body to intake plenum, and PVC lines, no rise in idle. the car idles great and runs great until the issue occurs. It is very intermittent. I also unplugged every coil and the coil harness to check for corrosion on the connectors they all look great. when the car is running great I have gone on the freeway and floored the car until I reach 80 MPH. I did this about 5 times. The car will put out some smoke but not excessive. The care never acted up during that time. I think it seems to be a loose connector. It has acted up when the car is cold but mostly after it has been driven for about 10 or 20 miles.
Customer reply replied 2 years ago
I also replaced the fuel filter
Answered in 15 hours by:
11/9/2015
Cadillac Mechanic: Pete, Cadillac Technician replied 2 years ago
Pete
Pete, Cadillac Technician
Category: Cadillac
Satisfied Customers: 32,211
Experience: 17 years Cadillac experience as a technician.
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My name is ***** ***** I am a professional here at Just Answer. I noticed that your question had not received a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.

In some cases this fault here may be caused by a lack of fuel through the fuel injectors or to much fuel through one or more injector intermittently.I have seen fuel injectors act up like this before intermittently with the affected injectors sometimes needing replaced.If you have not tried having the injectors cleaned I would see if this corrects the fault.You can also try and add injector cleaner to the fuel tank at around 1/4 tank and run to near empty to see if this clears up the injectors.

In some cases a restricted fuel filter or fuel pump that is acting up intermittently will also cause this type of fault here.If you have not had the fuel system tested I would see if it is at fault needing replaced to correct this issue as well.

All of these need checked into/tested to locate and repair the fault.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.Thanks Pete.

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Customer reply replied 2 years ago
Yesterday the issue occurred again. After having driven about 5 miles, I was driving along at 30 MPH the CEL came on flashing and the engine was very rough running. The car slowed to 15 mph. I pulled over and checked codes. Same p0300 and u1000. I opened the hood and pushed on the fuel pressure check valve. Fuel came out. Don't worry I had a towel around it. Then the engine started running proper and the CEL and codes went away. I drove it home with no issues and later drove it to the store 10 miles no issues. the car has 1/4 tank of fuel in it now. I am going to get some injector cleaner and see if I can get a loaner fuel pressure gage. I have to drive the car about 40 miles today. I will update when I return. The car has had 3 tanks of gas since this issue first occurred. I usually fill up at about 1/4 tank. I will drive it until the low fuel light comes on this time.
Customer reply replied 2 years ago
Fuel pressure at idle is 36PSI. If I rev the engine pressure rises to 40 then drops back to 36. is this pressure OK? also is the u1000 code irrelevant to this issue? I get a service stability system every time we get in the car. Could this be the source of the u1000 code and have nothing to do with the intermittent p0300. I also put a whole bottle of Lucas in the tank while 1/4 tank of fuel.
Customer reply replied 2 years ago
Key on and engine off the pressure is 40PSI? Running the pressure drops to 36. Is this normal?
Cadillac Mechanic: Pete, Cadillac Technician replied 2 years ago

Fuel pressure key on and engine off should be between 41 and 47 psi.Running you might see the lower end 36 to 41 psi so it looks o.k. right now but I would re check this when the vehicle is acting up as the pump may be providing pressure now but when the vehicle is acting up it may be lower which can indicate a fuel pump issue here.You may have some injectors that are gummed up.I would drive the vehicle with the cleaner in the tank to near empty and see if it clears up then at all.

U1000 will set when a loss of communication with one or more computers in the vehicle occurs.This won't cause the misfire and diagnostics say to diagnose and repair any other faults and codes first then see if this code returns again.

Thanks Pete

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Customer reply replied 2 years ago
The engine has not acted up for 35 miles. The Service engine soon light came on solid but car ran fine. I have the fuel pressure gage attached and positioned under the windshield wiper so that I can see it while I drive.I can see I now have a new code in history p0174,p0300 and current RIM u1016. Would replacing the fuel pressure regulator be worth the effort? I can get one for $45.
Customer reply replied 2 years ago
Interesting note. If I disconnect the vacuum line from the fuel pressure regulator the fuel pressure holds steady at 44PSI the whole time I drove it around the block. Is this an expected result?
Customer reply replied 2 years ago
I drove the car until the fuel was a low as I dared. I put another bottle of Lucas and filled up with Chevron Techron gas. The issue has not returned and the only current code is U1000. The other codes p0174,p0300 and Rim u1016 are history codes.
Cadillac Mechanic: Pete, Cadillac Technician replied 2 years ago

The fuel pressure will hold steady for the most part with the regulator disconnected and this is normal.

Since the fault has not re occurred here the issue may have been resolved.I would clear out the codes and continue to operate it to see if it acts up again at all.

Thanks Pete

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Customer reply replied 2 years ago
Drove the car 25 miles this morning. 16 Miles of freeway speed. On the return trip after exiting the freeway and coming to a stop light, the SES came on new code P0420. The other codes p0174,p0300 and Rim u1016 are still history codes.
Cadillac Mechanic: Pete, Cadillac Technician replied 2 years ago

O.k.,the code P0420 indicates a fault with low exhaust flow.Usually this is the catalytic converter that becomes restricted and usually due to a misfire with excess fuel that can build up in the catalytic converter.This may be the case here with yours needing replaced.It can also indicate that the rear oxygen sensor is faulty but in most cases it is the catalytic converter that needs replaced and is most likely the result from the misfiring that was occuring.

Thanks Pete

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