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1991 Cadillac Sedan Deville 4.9l 80k miles
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79 Posts
Discussion Starter #1
I have a 91 deville with a 4.9l. I have done a lot of diagnostic for a apparent miss. I had a tune up which was supposed to be new plugs, wires, fuel filter, cap, and button. That was a year ago.

Still, the miss continued. I speculated on this board about it possibly being the fuel pump. I checked the fuel pressure. It seemed normal. That being said I cant drive the car and look at the gauge cause the tester hose isn't long enough to tape to the windshield. It was said the coil might be an issue so i ended up replacing the whole distributor and internals. Still the miss continued.

The miss doesn't start at ignition. It seems to start at higher speeds like getting to 60mph on the interstate and works it's way all the way down to where traveling 20mph becomes very challenging. Its definitely happens quicker when the temperature out side is higher. When it gets that bad the car stahls easily without the accelerator. Now the kicker is if when it's at the worst if the car can set for say an hour. The miss is a lot better or gone for a while. The only other odd piece of information is when I totally fill the tank the miss seems to occur almost always.

I'm kinda to replacing the fuel pump. My concern is spending the money for a fuel pump and it ends up being the way more expensive sending unit.

Is it possible that there is something wrong with the plugs, wires, or fuel filter that would cause my problems. I'm not getting any engine codes. Have no idea if that helps or not but figured I'd say. Also, I had the catalytic converter tested and it was 100% fine. Please help!?!
 

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2001 eldorado,2008 DTS,2005 XLR, '96 eldo,'95 eldo,' 89 eldo,'78 eldo,'11CTS-V
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1,819 Posts
My first thought is fuel pressure. Clogged filter or sock on the pump...or a failing pump.
Somehow,get that pressure gauge hose lenghtened and taped to the windshield.
 

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Registered
1991 Cadillac Sedan Deville 4.9l 80k miles
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79 Posts
Discussion Starter #3
How many fuel filters are there. I'm thinking fuel pump because of the apparent temperature variable. Is there anyway that replacing the (I'm guessing external) fuel filter cause these issues? It was supposed to be replaced. Another fact is that this car does have the typical issue of stalling when fuel level is low and you brake and turn left. I've read that there is a baffle issue in this model. Can this be related. Also, the car starts to act up when electronic fuel system says it has 4 gallons left ie stalling, lurching, like it's out of fuel.
 

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2001 eldorado,2008 DTS,2005 XLR, '96 eldo,'95 eldo,' 89 eldo,'78 eldo,'11CTS-V
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1,819 Posts
How's it run with a full tank of gas?
Gas gauges of this period have been known to be erratic.
I've reset my trip odo at every fillup, and when it gets to be around 300, I'm probably runnng on fumes. Just a check on what the gas gauge reads.
As for the filters; one on the left frame rail, and one sock on the pump pickup.
Again, a fuel pressure gauge will nail a fuel delivery problem.
 

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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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19,724 Posts
I checked the fuel pressure. It seemed normal.
there IS a reason we ALWAYS ask for NUMBERS -
and "normal" is not a number -

fuel injectors were an issue -
use an ohm meter and measure the resistance of each one -
they SHOULD be virtually identical -
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,118 Posts
Another fact is that this car does have the typical issue of stalling when fuel level is low and you brake and turn left. I've read that there is a baffle issue in this model. Can this be related
It should not affect you when on the interstate. The baffle problem usually causes a stall when below 1/4 tank and making a turn.
 

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92 Fleetwood 2dr cpe - FWD, 96 Seville SLS, 02 Seville
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960 Posts
Most severe at 1/4, but will start acting up at 1/2 with sharp stops and jack rabbit starts.

You probably have a bad fuel injector. Known problem with the 4.9. On cold start, you are running open loop with the ecu running a higher idle until the car warms up. This extra idle is compensating for the missing cylinder (bad injector). Once on the road, where the power is needed, the missing cylinder becomes obvious. Now the car is warmed up, in loop, reduced idle so when you come to a stop, you are unable to maintain a good idle.

There is a power test via the diagnostics that allows you to deactivate each injector. Once deactivated, the idle rpm should drop 50-75. If it doesn't drop, then that injector/wire is bad.
 

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1991 Cadillac Sedan Deville 4.9l 80k miles
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79 Posts
Discussion Starter #8
I cannot find a complete guide on how to use the computer to diagnose problems. I printed a small post but it was about getting readings more than turning things off and on. Is there a post I can print with the complete guide. Also, I don't think the baffles are the issue at all. I was posting all information I had. The injector problem sounds solid but why does the problem go from accelerating above 60mph to not being able to go up hill 10mph. If I dont feather the pedal I cant go anywhere. It progresses kinda quickly after about 10 mph. The temperature variable is why I dont think fuel filters. Is it possible the fuel pump is overheating? And a dumb question, what's the fuel sending unit do? The fuel pump sends the fuel right?
 

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Registered
1991 Cadillac Sedan Deville 4.9l 80k miles
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79 Posts
Discussion Starter #9
I cannot find a complete guide on how to use the computer to diagnose problems. I printed a small post but it was about getting readings more than turning things off and on. Is there a post I can print with the complete guide. Also, I don't think the baffles are the issue at all. I was posting all information I had. The injector problem sounds solid but why does the problem go from accelerating above 60mph to not being able to go up hill 10mph. If I dont feather the pedal I cant go anywhere. It progresses kinda quickly after about 10 mph. The temperature variable is why I dont think fuel filters. Is it possible the fuel pump is overheating? And a dumb question, what's the fuel sending unit do? The fuel pump sends the fuel right?
 

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70 Deville 78 Seville 92 Deville 03 Deville
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780 Posts
None of this sounds fuel pressure related to me. If you don't want to pay the big bucks for a FSM you can buy a CD version or download version for about $15. That will tell you everything you need to know about the OBD. I'm wondering if your catalytic is clogging. A simple vacuum test would verify that.
 

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1991 Cadillac Sedan Deville 4.9l 80k miles
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79 Posts
Discussion Starter #12
Cat has been tested not clogged no pressure. when you refer to an FSM do you mean like a Chilton's. If not where do I find it?
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,118 Posts
when you refer to an FSM do you mean like a Chilton's. If not where do I find it?
No, Chilton's and Haynes are pretty much useless for all but the most basic things.




Several more on Ebay
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
Joined
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19,724 Posts
I cannot find a complete guide on how to use the computer to diagnose problems. I printed a small post but it was about getting readings more than turning things off and on. Is there a post I can print with the complete guide. Also, I don't think the baffles are the issue at all. I was posting all information I had. The injector problem sounds solid but why does the problem go from accelerating above 60mph to not being able to go up hill 10mph. If I dont feather the pedal I cant go anywhere. It progresses kinda quickly after about 10 mph. The temperature variable is why I dont think fuel filters. Is it possible the fuel pump is overheating? And a dumb question, what's the fuel sending unit do? The fuel pump sends the fuel right?
that is all listed in the FSM - Factory Service Manual -

I found one for you - $23 - including shipping -

BEST $23 you'll EVER spend -
 

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1991 Cadillac Sedan Deville 4.9l 80k miles
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79 Posts
Discussion Starter #17 (Edited)
I got an e31 code for map sensor. I have already replaced the map. Any ideas? My battery never seems dead but there seems to be a draw because when I hook up the charger it never is at 100%. Could a less than 100% battery cause my problems?
 

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92 Fleetwood 2dr cpe - FWD, 96 Seville SLS, 02 Seville
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960 Posts
Pigtail/wire harness is probably gone. Also, vacuum hose might have a leak.

Here is level one of the diagnostics. P02 will show what the map sensor is reporting.

Cadillac diagnostics
To enter the diagnostics system, simultaneously press and hold the OFF and WARMER buttons with the engine running until the system activates. Any diagnostic codes will display first. When the system prompts you with .7.0 you want to press the LO button. This will force the sytem to the PCM data mode and the display will show E.9.0 Use the HI and LO buttons to scroll thru the PCM data parameters. Here is what you can access....

P01 throttle position in degrees
P02 MAP (manifold absolute pressure) in kPa
P03 computed barometer in kPa. Subtract MAP from BARO to get engine vacuum
P04 coolant temperature in degrees C
P05 MAT (manifold air temperature) in degrees C
P06 spark advance
P07 battery voltage
P08 engine RPM....tach
P09 vehicle speed MPH

P12 injector pulse width
P14 oxygen sensor voltage
P16 oxygen sensor cross counts...a measure of O2 sensor activity
P18 closed loop integrator (128 ideal...less equals leaning, more enriching)
P20 closed loop block learn value (same as above)



When driving with the diagnostics activated the icons on the climate control panel stand for other functions....kind of like tell tales on an instrument cluster....

rear defog is the VCC status
front defrost is 4 gear status
ECON is the o2 sensor status (on is rich)
AUTO is the closed loop status
OFF is the closed throttle switch status
OUTSIDE TEMP is the AC compressor request
F is the AC low pressure switch status
C is the heater water valve status
AUTO FAN is the cooling fan command status
 

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1991 Cadillac Sedan Deville 4.9l 80k miles
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79 Posts
Discussion Starter #19
I was driving while reading p02. It clearly varied between 26 at idle and 93 while about 55mph and missing. Also while in that mode I noticed in the eatc the econ and auto lights were off and on at random. What does that mean?
 

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Dear Friends, I had a similar problem with my 1988 Deville, 4.6 Liter. My mechanics finally figured out that there was a gap in the gasket at the injector housing. Does that make sense? This was several years ago. Air was leaking in and messing up the fuel air mix. The repair was cheap
Big Red R F.JPG
as I recall, and the car has run like a champ since then. Oh, I just got the horizontal surfaces painted and the rest of the body polished. Man, does it look good! I wish mom were around to see it, as Big Red was her car. The silver 2008 in the background is my daily driver. Ed
 
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