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Discussion Starter #1 (Edited)
Alright y'all. I'll start by saying that I am no mechanic and I'm learning as I go how to work on and diagnose issues on cars.
Now, to my issue.
I have a 1990 Cadillac DeVille with around 145,000 miles. I have owned it for exactly a year. The car has started having a severe loss of power. The car starts with no hesitation and will run fine until warmed up. Once warmed up it will briefly run normal then loses power. I'm talking pedal to the floor and it totally bogs down. I have checked what I thought was rational to check but had no luck.
I have changed the spark plugs.
I have checked the fuel pressure with a guage and they checked out to be in spec.
40 psi ignition on/engine off
34 psi ignition on/engine on at idle.
I tried fresh high grade gas.
I put a new fuel pressure regulator and fuel filter on it.
Cleaned the throttle body.
Checked for any loose lines.

_ _ _
Unrelated parts changed when I purchased the vehicle:
It needed a throttle position sensor and an egr valve.
Both of these were replaced and the car was running fine. The current issue has only been happening in the last 3 months or so.
Oil changed regularly and I take care of the car. I LOVE this vehicle. I have read various posts here and elsewhere saying possible MAP sensor, clogged cat, idle air control valve ecm....

I'm about to pull my hair out, and I'm bald already so...

Thanks for your time.
 

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'89 Sedan Deville
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28 Posts
Did you check your vacuum? Although usually with bad vacuum it'll run rough from the start, it wouldn't hurt to check. I know when I change my EGR I ripped the vacuum line behind it. I ended up replacing most of the lines after that. Check my history, I also made a post about changing the vacuum canister intake line. Check visually for cracking and brittleness. You can also smoke test the system. A couple feet of 5mm silicone tube will take care of most of it.
 

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Discussion Starter #3
This may not be what you mean, but I checked all the lines to the throttle body and breather box and they were sucking.
 

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White Diamond '03 DHS (with DTS floor shift)
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I'm with Harry. It has all the symptoms of a clogged CAT.
 

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Discussion Starter #7
Thanks for the input guys. I was leaning more toward a clogged cat as well based on research I've tried to do. I wanted to ask someone who knew what they were talking about before I bit the bullet and ordered the cat. I'm very grateful for your replies, especially so quick. I'll order a cat today and post my results in the coming weeks.
Funny I'm ordering a cat and going cat fishing today as well. 😂
 

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92 Fleetwood 2dr cpe - FWD, 96 Seville SLS, 02 Seville
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956 Posts
Also, if Running the original injectors, they may be failing. There is a power test cuz the diagnostics that allows to shut off injectors one at a time while monitoring the rpms. If no rpm change, then failed injector.
 

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70 Deville 78 Seville 92 Deville 03 Deville
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776 Posts
Clogged cat, if you had a vacuum gauge you would most likely see the very low vacuum.
 

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So whats the verdict? Cuz I have a 90 Deville with SAME problem. Wanna know where i should start. (MAP sensor, Idle air control valve or clogged CAT)
 

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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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19,658 Posts
So whats the verdict? Cuz I have a 90 Deville with SAME problem. Wanna know where i should start. (MAP sensor, Idle air control valve or clogged CAT)
==========================
So whats the verdict? Cuz I have a 90 Deville with SAME problem.
while you MIGHT have the same SYMPTOM -
you may - or may not - have the same PROBLEM -

you need to DIAGNOSE your PROBLEM -

STEP #1 - ALWAYS - check for DTC's - Diagnostic Trouble Codes - using the on-board scanner -
if you don't know how - go here -

post any codes you have -
ALONG WITH THEIR DEFINITIONS -

include whether they are C-Current or H-History -
 

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Alright y'all. I'll start by saying that I am no mechanic and I'm learning as I go how to work on and diagnose issues on cars.
Now, to my issue.
I have a 1990 Cadillac DeVille with around 145,000 miles. I have owned it for exactly a year. The car has started having a severe loss of power. The car starts with no hesitation and will run fine until warmed up. Once warmed up it will briefly run normal then loses power. I'm talking pedal to the floor and it totally bogs down. I have checked what I thought was rational to check but had no luck.
I have changed the spark plugs.
I have checked the fuel pressure with a guage and they checked out to be in spec.
40 psi ignition on/engine off
34 psi ignition on/engine on at idle.
I tried fresh high grade gas.
I put a new fuel pressure regulator and fuel filter on it.
Cleaned the throttle body.
Checked for any loose lines.

_ _ _
Unrelated parts changed when I purchased the vehicle:
It needed a throttle position sensor and an egr valve.
Both of these were replaced and the car was running fine. The current issue has only been happening in the last 3 months or so.
Oil changed regularly and I take care of the car. I LOVE this vehicle. I have read various posts here and elsewhere saying possible MAP sensor, clogged cat, idle air control valve ecm....

I'm about to pull my hair out, and I'm bald already so...

Thanks for your time.
I am not a mechanic, i have 90 deville, no power. I did everything, tune up, rear cat, fuel filter, i changed all injectors and boom runs like a top. I also used generic injectors as i could not afford AC delco.
 

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Discussion Starter #14
So whats the verdict? Cuz I have a 90 Deville with SAME problem. Wanna know where i should start. (MAP sensor, Idle air control valve or clogged CAT)
Well I changed the cat and still have the same issue. I guess I'll order fuel injectors. No codes are showing
 

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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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19,658 Posts
Well I changed the cat and still have the same issue. I guess I'll order fuel injectors. No codes are showing
=========================
No codes are showing
codes don't just "show" by themselves -
you must get into the built-in scanner and EXTRACT the codes yourself -
 

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Discussion Starter #16
Well yeah, I'm saying when accessing the onboard diagnostic feature via the AC control panel, there are no codes shown.
 

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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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19,658 Posts
double check that fuel pressure -
spec should be between 45 and 50psi -

connect a fuel pressure gauge and observe the readings while driving -
 
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