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1991 DeVille code help


Devin O'Conor

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I recently purchased a 1991 DeVille. The other day, it stalled while I was turning a corner. Since then , it has been running rough. Here are the codes I am getting:

E71 - Intermittent MAP Signal

E98 - High RPM P/N to D/R Shift Under ISC

Does this just sound like a bad MAP sensor or should I look at something else? That would be dandy if I could replace the MAP sensor and be on my way but my mind tells me nothing works that easy. Any ideas? Anyone else had this problem? Thanks for any help.

K.O.T.

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Check the electrical connection at the MAP sensor - unplug it, plug it back in and maske sure it is fully seated. That might be enough to clean the contacts.

The high idle may be the result of an idle speed control motor failing (the closed throttle position switch). Do a search in the archives on "idle speed control" or "ISC" and read up. There is a ton of information there.

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

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I am not sure what symptoms an intermittent MAP signal will cause but I suspect it will cause a drivability problem. I would check the connections to be sure they are clean and tight. If that does not help, maybe unplug it and see if the symptoms are the same. That would pretty much point the finger to the MAP sensor. Then replace it.

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Thanks for the replies. Last night, I checked the ISC motor and found it needed an adjustment so I did that. I also removed the map sensor and then reinstalled it. After that, the problems seem to have disappeared. The idle smoothed out and it didn't seem to want to stall anymore. Today I have about 200 miles to drive and will report back on how it behaves today.

K.O.T.

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One more to watch for. That car had a poorly desinged fuel pick-up tube in the fuel tank. If the vehicle is low on fuel and you turn a corner, gasoline does not get into the pick-up tube and will cause the vehicle to stall. This only happens when the vehicle is low on fuel and you are turning a corner. The replacement part for this has been redesigned so once it is changed, you will not have the problem anymore.

Max

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Well, today the car received a new MAP sensor. The problems seem to be ocassional and the new MAP sensor didn't help. This evening I was making a left turn and it bogged down for about one second and picked back up. It stalled earlier today but that was just before the new MAP sensor was installed.

The two times it stalled, I was low on fuel but this evening, when it bogged for a second, and almost stalled, it showed I still had 8 gallons in the tank. A while later after the car had sat for about 30 minutes, I started it and the idle went up and down like it was searching for the right idle speed. It's possible that these problems are worse when it is colder in the evening.

Any ideas? Does any of this sound like a possibly failing TPS?

Thanks.

K.O.T.

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You need yo hook up a fuel pressure gage to the fuel rail and tape the gage to the windshield. When the stalling occurs, observe the fuel pressure to see if it drops off.

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

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You need to hook up a fuel pressure gage to the fuel rail and tape the gage to the windshield. When the stalling occurs, observe the fuel pressure to see if it drops off.

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

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I don't have access to a fuel guage right now. I did install the new ISC motor today and so far it seems to be helping. I think I have it properly adjusted. It doesn't make the "ratcheting" sound anymore like the old one did. I also installed a new coil, cap, and rotor. It really needed them I think. The car seems to have picked up quite a bit of power with them.

I will drive the car for a few days and report back if the new ISC motor did indeed repair the problem.

Thanks for the help!

K.O.T.

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Devin,

You never mentioned throttle body. Have you cleaned it up really good? If the bores and blades (particularly back sides) are not like new the motor will stall at low RPM. Pay also attention to two EGR tubes inside the TB. Clean everything with some carb. cleaner, toothbrush, etc. A rattling ISC motor is definitely defective, that's good you got rid of it , but new one should be carefully adjusted. You can find all the info you need in recent and old archives.

The saddest thing in life is wasted talent

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I haven't had a chance to clean the throttle body yet. I think it really needs it also. It still almost stalled once yesterday even though it is running much better. Hopefully the TB cleaning will do the trick. I did thoroughly adjust the ISC motor. I need to check it again to make sure it is right. Can a dirty throttle body cause a rough idle? It is normally very smooth but every once in a while it seems rough while sitting at a stop sign.

K.O.T.

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I haven't had a chance to clean the throttle body yet. I think it really needs it also. It still almost stalled once yesterday even though it is running much better. Hopefully the TB cleaning will do the trick. I did thoroughly adjust the ISC motor. I need to check it again to make sure it is right. Can a dirty throttle body cause a rough idle? It is normally very smooth but every once in a while it seems rough while sitting at a stop sign.

It makes sense to adjust ISC motor AFTER TB is parfectly clean. When it is clean disconnect the battery for a minute or so to reset the default settings.

The saddest thing in life is wasted talent

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Can a dirty throttle body cause a rough idle? It is normally very smooth but every once in a while it seems rough while sitting at a stop sign.

I would not expect a dirty TB to cause a rough idle. High idle maye, but not rough.

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Mr. O'Conor;

In our type of Caddies, the TB is GOD!!! hehehe...

Clean it up with the care of a surgeon and she will run better guaranteed.

I had similar issues, spent hours and hours and $$$$ and found she just needed some intense TLC in the TB.

Good luck...

Mike P

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That car had a poorly desinged fuel pick-up tube in the fuel tank. If the vehicle is low on fuel and you turn a corner, gasoline does not get into the pick-up tube and will cause the vehicle to stall. This only happens when the vehicle is low on fuel and you are turning a corner.

Maximln, you were right. The car was stalling from the fuel not getting in the pump while turning. It never stalled unless I was turning and was always with 7 gallons or less of fuel. I will just have to keep it above half a tank until it needs a new fuel pump. I probably won't be replacing it since it still works.

As far as the rough idle, I thoroughly cleaned the throttle body. It shines now and it certainly did help the rough idle. I also disconnected the battery to clear codes and also to let the computer re-learn idle. The car runs perfectly now.

Thank you all for all your help!

K.O.T.

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Devin,

As you see multiple suggestions allowed you to fix ALL the problems you had had.

To make the motor run even smoother you might want to replace the plugs somewhere in the spring. ACDelco regular plugs are inexpensive and very good. See my recent post:

http://caddyinfo.ipbhost.com/index.php?sho...c=3733&hl=plugs

The saddest thing in life is wasted talent

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I do plan on replacing the plugs and wires very soon. I thought the 4.9 came from the factory with double platinum plugs. Did they not? If not, mine will be well worn also with 134,000 miles.

I'm not looking forward to replacing the rear plugs.

K.O.T.

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I do plan on replacing the plugs and wires very soon. I thought the 4.9 came from the factory with double platinum plugs. Did they not? If not, mine will be well worn also with 134,000 miles.

I'm not looking forward to replacing the rear plugs.

I am pretty sure 1991 models were not equipped with double platinum. Stick with regular Delco and you'll be fine for another 10 years, they are more reliable than platinum ones ( at least they do not have that pad to loose). I did not find replacing rear plugs more difficult than the front ones. The hardest part is pulling the boots off. Turn them 1/2 turn before pulling. Deep throat socket with 6'' ratchet handle worked fine. As far as I remember the most difficult to work on was the left front plug, some AC line did limit the access.

The engine runs definitely smoother with new plugs, but I did not notice any significant gain in performance. Just one observation - I retracted ISC motor plunger completely to let the engine idle at minimum air RPMs, and the engine stalled at some 500 RPM. The coolant temp was around 100 F and it was old set of plugs. I repeated the experiment with new plugs installed (same part number) and the engine DID NOT STALL at 500 RPM even the temperature was 40 F! That's the difference the new plugs made.

I encourage you to replace ALL plugs. After I scratched badly my hands while working on front ones I really did not care much anymore and went ahead with rear row.

The saddest thing in life is wasted talent

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  • 2 weeks later...

Well, I replaced the plugs and it ran worse than before. So, I figured it was a bad spark plug wire. I replaced them and it is still bad. So, I purchased another set of plugs and hopefully it will be because of a bad plug.

If another set of plugs doesn't do the trick, I'm not sure where to go from there. I have heard the 4.9 has weak injectors. How does the 4.9 act when it has a bad injector?

The reason I think it could be a bad plug is because when I pulled the original plugs, one of them smelled pretty strongly of fuel. I figure that cylinder wasn't firing. Also, in the last few days, I believe the oil is getting thinned down with fuel.

Any ideas? Does it sound like maybe one or more of the new plugs is a "dead" plug?

K.O.T.

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Bad new plugs would be the last thing I would suspect... Did you use some dielectric grease to ensure good long term contact? I would not replace NEW plugs with another set of new ones. The problem is somewhere else. Well... replacing plugs never hurts, do not get upset.

The saddest thing in life is wasted talent

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The new plug wires came with di-electric grease in the boots. I am going to replace the plugs today on the off chance that one or more is bad. If that doesn't do it, I guess I will have to start testing injectors.

Anyone ever had an EGR valve go bad on a 4.9? How do they act in this instance?

K.O.T.

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