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Faltering acceleration


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First let me say that it was a God-send to find this forum. It is great and the participants are great! I'm 73 and a former (much former) shade-tree mechanic. As a volunteer at a non-profit I help other volunteers with their minor car problems.

I got my 1991 Deville last year with only 87000 miles on it. It is now 100k. Ever since I've had it the following occurs.

On our freeways it runs fine most of the time. Off the freeway, after driving for about a half hour, either in traffic or on a straight road, it will seem to falter when I try to accelerate or am climbing a hill. (This can sometimes happen on a short freeway drive also.) On a carburated car I would say it is because the fuel-air mixture is not correct when pressing down the gas pedal. I'm not that familiar with the newer engines (4.9 on this one) so I am "befuddeled". I don't know if it is "carburated" or not. A computer check showed no errors.

My solution has been to just accelerate slowly when it starts to happen but that can be a problem when someone behind me wants to get up that hill faster!

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Welcome to Caddyinfo.

Did you check the basics, such as plugs, wires, air filter, and did you check the battery's connections to make sure there's no acid buildup or corrosion causing a bad connection?

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I bet on plugs/wires, if they haven't been replaced at some point in its life, just being that old could certainly cause what your describing.

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My '92 Deville (4.9) had a similar problem. Ran fine for the first :15 -:20 min. After that if I tried to got WOT it fell on its face, much like a bad accelerator pump on a carburated engine (the 4.9 is multiport fuel injected by the way), but ran fine at cruise. It turned out to be a failing fuel pump. A fuel pressure guage taped to the windshield showed the fuel pressure fall off at WOT. Just a thought.

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I agree with Larry - get a fuel pressure gage hooked up and duct tape it to the windshield and watch the fuel pressure reading when it acts up,

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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My first thought was the fuelpump which I then changed. No difference was noticed. Thanks for all of the inputs though. I'll change the plugs next week to see if that is part of the problem. It looks to me llike the rear plugs aregoing to be a serious problem though.

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I'm thinking filters here. There are at least two, one on the fuel line heading to the engine, and a sock filter in the tank. The sock filter can be a nasty one to diagnose since it will not be plugged all of the time. It only pull enough dirt in after long periods of running, then it plugs, then it falls on it's face during period of high fuel demand. And I believe your symtoms appear at times of high fuel demand, and something is not leting the engine get enough of it. Fuel pressure regulator bypassing too much? (stuck open)

Never underestimate the amount of a persons greed.

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My first thought was the fuelpump which I then changed. No difference was noticed. Thanks for all of the inputs though. I'll change the plugs next week to see if that is part of the problem. It looks to me llike the rear plugs aregoing to be a serious problem though.

Sounds like wire/plug problem to me. The last thing I would replace is the fuel filter. I have read DOZENS of posts where the authors had replaced fuel filter to fix something and cannot recall ANY post where it would make difference. Looks like the fuel got much cleaner nowadays.

The rear plugs on 4.9 are actually easier to replace since there is more room up there compared to front ones. Just do not be shy, lay across the engine. You do not need too see them to replace. Just reach them by following the wires.

The saddest thing in life is wasted talent

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  • 2 weeks later...
  • 9 years later...

I'm a new poster. I have a 1991 Cadillac Fleetwood Brougham with a 5.0 L motor and has 119,000 miles on it.The car was running great until about a month ago on a trip I started climbing a hill and my speed started coming down. It kicked down to the next gear but this did not help because it continued Bogging down. It then got to the point I could not go over 55 mph without it bogging down. A while later it went to 45 mph, then 30 mph etc. etc. It sounds like the fuel air mixture is not working properly because you can hear the air get sucked into the carburetor. I removed the air cleaner and watched my injectors while opening the throttle up. They start adding fuel as they should but if you kick the throttle down about halfway it bogs down. I changed the fuel pump, in-line fuel filter on the back frame, the throttle position sensor, rebuilt the TBI and cleaned the injectors at the same time. I pulled vacuum on the EGR valve and found that the diaphragm is leaking. I plugged vacuum input to the EGR since this should not affect this problem. Now the car idles fine but as soon as you press the accelerator down a little it starts bogging down and it will die if left in a position. If anybody has any ideas of how to fix this problem it would be greatly appreciated.

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Welcome ! Do you have a way to read the codes (DTCs) from the computer? If you don't have a code reader, and the Fleetwood doesn't display them on the dash, often you can go to a local auto parts store and they will read them for you. Of course they also sell code readers. It definitely sounds like you are getting too much fuel, or too little air. Could your exhaust be clogged?

Bruce

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Definately check your codes but this sounds like a clogged CAT converter

This type of gauge screws into the O2 sensor port and will indicated excessive back pressure

http://www.amazon.com/gp/aw/d/B0002SQW9S/ref=mp_s_a_1_1?qid=1465163146&sr=8-1&pi=SY200_QL40&keywords=exhaust+back+pressure+tester&dpPl=1&dpID=41NROkvMXgL&ref=plSrch

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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If its like my sts, you can unbolt the 4 bolts and separate the catalytic convertor from the exhaust and you will feel the performance improve immediately it thats the problem.

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Simultaneously press and hold the OFF and WARMER buttons on the climate control panel with the key in the ON position. Any trouble codes will be displayed on the climate control panel.

In my opinion, your symptoms sound like a plugged catalytic converter.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Power brake it and have someone "feel" the exhaust flow. If it falls off to hardly any flow it is the cat.

Tap on the cat with a rubber mallet when it is cooled off and listen for a rattle, if it sounds like marbles are inside - its bad. OR, pull it off if it's an easy one to remove ( NOT rusted bolts ) and look through it, if the honey comb is "melted" or you can't see light through it it's bad.

THERE IS ALWAYS ENOUGH TIME TO DO THE JOB RIGHT - THERE IS NEVER ENOUGH TIME TO DO THE JOB AGAIN !!!

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Welcome ! Do you have a way to read the codes (DTCs) from the computer? If you don't have a code reader, and the Fleetwood doesn't display them on the dash, often you can go to a local auto parts store and they will read them for you. Of course they also sell code readers. It definitely sounds like you are getting too much fuel, or too little air. Could your exhaust be clogged?

Thank you for the welcome, and let me tell you I like this board. Anyway, since I last posted I checked the map sensor and a check tell good. Unfortunately during testing I had to disconnect the negative battery terminal, hence, bye-bye codes. After driving it a while at 10 miles an hour I now read codes 00 and 48. I don't show 48 in my manual but an E48 calls out a bad EGR valve which I already knew. I was going to check the exhaust back pressure next but got too frustrated after working on this for three days LOL. The car has been in the shop and being worked on since 8:30 AM. The mechanic tells me his OBD1 will not read the codes, so I told him how to do it through the panel. He took it out for a drive at 3 PM and nothing has changed still can't push pedal more than 1/4 inch before it bogs down. My other code was 00 and does that mean no symptoms? I'll post back what the mechanic has found if he don't kill me first for bringing this car in LOL. I also told him to try disconnecting the exhausted at the middle and take it for a drive.

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That E048 could be triggered by the clogged CAT as the exhaust is forced back through the EGR due to the backpressure.

I think you are going to find a clogged CAT

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Possible clogged cat or a timing issue. Check your ignition timing, disconnect a single wire black and tan I believe near your distributor and set base timing. The specs should be on your fan shroud.

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Disregard the above, I thought you had a 90 Brougham. Check your timing though by jumping the aldl connector where the heater vents are under the dash. You know the two terminals are jumped correctly when the engine stumbles for a second and the service soon light is on.

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Well I finally got my Cadillac back from the mechanic. His OBD1 could not read my computer so he just manually checked everything including a fuel pressure check which read 6 psi. After about four hours he taped a pressure gauge to the windshield to see what's happening when it starts losing power. Told me I needed a new fuel pump because the pressure went from 6 psi down to nothing after about three minutes. This pump is only about eight weeks old. Luckily it had a one year warranty. I bought another pump and installed it. When done, I could hear the pump running but nothing was coming out of it. I called AutoZone and asked if maybe the pump had to be primed or something. They told me no, once installed it should just start pumping fuel out. They asked how much gas was in the tank. I told them about 5 gallons which should be plenty for the pump to work even with the backend on ramps. He told me to try adding 5 gallons and see if it worked. So I added 5 gallons, put to connector together and got a few dribbles out of the pump. Not only do I have to drop the tank again but it's also got 10 gallons of fuel in it now. So in the morning I'm going to drop the tank once again, pull the pump out and bring them both back to AutoZone and get a refund. Then I'll go to Napa and buy a quality pump. I'll let you know how everything turns out.

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The worst kind of problem is when you replace a part and its defective because its eliminated from the list of possibilities.

Good luck, let us know how this turns out

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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The fuel pump should pump fuel right down to the point of the tank being empty. Autozone does not have the beat quality parts. Don't let some Autozone clerk convince you that you need at least five gallons in the tank. I'd change the pump out with an AC Delco and you will probably never have to do the job again

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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Well I dropped the tank AGAIN and put a good fuel pump in. Keep in mind this car has been sitting for five years. I started the car and it would idle for a few seconds and die. So I started checking the fuel delivery line and when I disconnected the feedline from the tank and the in-line filter the flow was very weak. Went to Napa and got a new line between the tank and a filter and installed it. Now I have six PSI to the throttle body input. Now it will idle with a lope and if I keep pushing the throttle down the RPMs start to come up but something is still trying to kill it. I have no vacuum leaks in his motor. Could it be after sitting for five years that the GP Sorensen injectors are beyond cleaning and they're just not putting out fuel requested? I checked the map sensor, throttle position sensor and the coil, they are all good. Within a few days ago I checked the codes at the temperature control panel and got 00, 48 and the new one 132. I checked them today and 132 is gone. I guess I don't have to tell you folks I'm ready to pull my hair out, what little I have left LOL

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