: 93 DeVille 4.9 Questions

11-17-12, 02:00 PM
hi all I've been using your forums for a while now I just decided to register! I just got a 93 Deville last week and i am curious. I am getting about 13.5 miles to the gallon right now, and I'm wondering if that is normal. it is city driving but I have been driving the car like a grandma. I getting the E39 code, which i know will cost you a few mpg but i didn't think it was that drastic! I also am getting the E44, and my oxygen sensor looks like the original. This car has 155,000 miles. I already checked for vacuum leaks and cleaned the Egr tubes, I also checked the ground wires that go to the block. The car doesnt miss and it runs strong, its very fast. So i am baffled as to whats killing the mpg. Also i've been using premium gas, and i changed the fuel filter, but no change. Any help would be appreciated thanks!

11-17-12, 04:47 PM
I'm getting about 16 mpg with my 92 eldo. 4.9, dont waste your money on premium gas, you dont need it, I'm on my 3 rd. 4.9 and have always used 87.

11-17-12, 05:39 PM
Ok. Ill try 87 again. I guess my question is this: How probable is it that i need a new O2 sensor. At this point with the E44 and no other actual causes for a lean condition. Can I bet on this or do you think that is a waste of time. The only reason I'm asking is because I have spent hours on this forum and i either get conflicting opinions or the op never lets you know how they resolved the problem.

11-18-12, 09:04 AM
I guess if you are getting a code for O2 it must need to be changed. The most important things affecting mileage would be FPR or MAP or O2 sensors. I'm not sure right now if these cars have a MAT sensor too; it has to have one.

11-18-12, 12:16 PM
I guess if you are getting a code for O2 it must need to be changed. The most important things affecting mileage would be FPR or MAP or O2 sensors. I'm not sure right now if these cars have a MAT sensor too; it has to have one.

Yes Al, they have a MAT (intake manifold air temp sensor). Al is correct on the FPR MAP and O2 sensors. You also need the coolant temp sensor in correct working order. Coolant temp can affect whether your engine goes into closed or open loop. There will always be the debate on premium vs regular gas. Your engine was designed to run on premium. You can retard the timing a few degrees to help reduce pinging, but will notice a decrease in performance.

A code E39 will definately hurt highway mileage. Don't be surprised if you have other transmission issues.

A code E44 can be from 2 problems. 1. There can be a circuit problem in the O2 sensor 2. There can be a lean condition from a vacuum leak or fuel system control fault.

Your problem needs to be diagnosed properly before you go throwing parts at it. I would recommend getting a can of throttle body clean with the tube on it, spray on and around vacuum lines. If your idle increases, then you found a vacuum leak. With the age and mileage of your car, it wouldn't surprise me if you have several vacuum hoses that need replaced.

11-19-12, 09:22 AM
Ok thanks guys for all your help. I wanted to let you know I have already repaired all the vacuum leaks as there were a few due to those plastic lines being split. I checked the fpr, at idle I disconnected the the vacuum hose and i didnt see any fuel and there was a good change in the way the car ran. I used to have an egr code, but After fixing hoses that went to the egr, that went away and seems to be in good operating order. According to the diagnostic parameters, my MAT is just under 100 degrees and oolant temp is around 210, this is after converting to fahrenheit from celcius. This leads me to believe that it is the o2 sensor which looks original. I dont mean to waste your guys time, I just wanted to get suggestions before throwing money at it. That sucks about the tranny, this is the best shifting cadillac ive ever had. Also, since ive dropped the pan and filter on the trans on the highway the instant display reads 25-29 mpg when on the highway, but I'm not sure how accurate that is and I'm not on the highway enough to do my own figures right now. Thanks again in advance

11-20-12, 08:58 AM
What does your parameters show for O2 voltage? It should be swinging from a lower voltage to a higher voltage. Also should tell you the number of crossover counts.

11-20-12, 10:49 AM
The O2 voltage does seem to change, but the cross counts usually stay at 0, 1, or 2. They eventually start to go up, but then drop back down under 3. It was my understanding that it was supposed to be greater than 3. Also, the Voltage is changing like I said, but not a whole lot. If you floor it it only moves up a couple hundredths, but I'm not sure if the change is supposed to be drastic or not.

11-20-12, 01:34 PM
the change should be relatively drastic. Yoiu should be able to see voltages as low as 0.10 and as high as 0.90. If it's barely bouncing around 0.45, it's probably no good.

11-21-12, 03:52 AM
If you are doing a lot of short trips and stop and go driving then yes that 13.5mpg can be normal. Reset the average too see what mileage you are getting, instant doesn't mean very much.

92 4.9 Deville
12-07-12, 01:54 AM
Man I wish I was getting 13.5. Since my gas experience the other night I am only getting and average of 10.3-10.7 city as of tonight which is up from yesterday at 9.5-10.2mpg. I have no codes set.