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Hello there!

I have had this car for three years now and it has been absolutely great. Never had a problem with until now. It has about 150k miles on it (I personally put 30k on it). I had it serviced to change the water pump that was making unhappy noises. Then, for some obscure reason, I turned the car on and the engine seemed to be misfiring at least 3 (maybe four) pistons. I took it to my mechanic and after a full week of tuning things up (standard spark-plug/coil/cables replacement) it only has a single piston misfiring (number 8 if I am not mistaken). My mechanic gave up and told me he couldn't figure out what as wrong with it.He ran the fuel injection cleaner for a while but no success. I really want to keep this car (it belonged to my grandfather). Does anyone have any idea of what steps I can take to try and fix this? (I am no mechanic but I can follow instructions).

Thank you!
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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21,268 Posts
Hello there!

I have had this car for three years now and it has been absolutely great. Never had a problem with until now. It has about 150k miles on it (I personally put 30k on it). I had it serviced to change the water pump that was making unhappy noises. Then, for some obscure reason, I turned the car on and the engine seemed to be misfiring at least 3 (maybe four) pistons. I took it to my mechanic and after a full week of tuning things up (standard spark-plug/coil/cables replacement) it only has a single piston misfiring (number 8 if I am not mistaken). My mechanic gave up and told me he couldn't figure out what as wrong with it.He ran the fuel injection cleaner for a while but no success. I really want to keep this car (it belonged to my grandfather). Does anyone have any idea of what steps I can take to try and fix this? (I am no mechanic but I can follow instructions).

Thank you!
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Does anyone have any idea of what steps I can take to try and fix this?
YES -
STEP #1 - ALWAYS - check for codes using the built-in scanner -

some issues aren't monitored - so they will NOT set a code -

inspect/clean the TB - Throttle Body -
make sure the EGR tubes in the base are open and clean -

check the fuel pressure at the fuel rail -

weak/failing fuel injectors are a fairly common problem in the 4.9 motor -
with the engine running -
use a stethoscope - or similar - and listen to each fuel injector -
you should hear a sharp - very distinctive SNAP every time the injector fires -
note any difference in sound between the 8 injectors -

another injector test is to measure the resistance of each injector -
they should all be nearly identical -

if you need new injectors you should get the up-graded 4-hole units -
a MATCHED set of 8 - remanufactured BOSCH injectors run around $100 - with free shipping on ebay -

small vacuum leaks can cause a slight misfire and finding them can drive you NUTS -
with the engine running -
use a can of carb cleaner with the little straw attached - and spray around each hose -
if the idle speed suddenly increases - you found a leak -

ignition timing is very sensitive on the 4.9 - assuming you use PREMIUM gas -
 
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