Author Topic: 25 RPM drop on '91 4.9 Engine.  (Read 708 times)

Offline Barry M Wheeler #2189

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25 RPM drop on '91 4.9 Engine.
« on: October 30, 2018, 12:58:18 PM »
The Seville is running pretty well now. I went on a trip last week to LaPorte (nearly 200 miles) so I could ride to a meeting with Lars Kneller. It's running 22.5 MPG and the tach is @ 900 RPM  at idle except for a cycling drop to 875. One injector blocked or ???
Barry M. Wheeler #2189


1981 Cadillac Seville
1991 Cadillac Seville

Offline Dave Shepherd

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Re: 25 RPM drop on '91 4.9 Engine.
« Reply #1 on: October 30, 2018, 06:19:59 PM »
Is the rpm drop connected to a misfire?  Also try cleaning the throttle body, it can get carboned up and affect the idle.

Offline Barry M Wheeler #2189

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Re: 25 RPM drop on '91 4.9 Engine.
« Reply #2 on: October 30, 2018, 08:28:50 PM »
It doesn't seem like it's misfiring as it has plenty of power and doesn't stumble regularly. (Every now and then when I'm pulling away from the house the first trip of the day it gulps once.)
Barry M. Wheeler #2189


1981 Cadillac Seville
1991 Cadillac Seville

Offline Dave Shepherd

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Re: 25 RPM drop on '91 4.9 Engine.
« Reply #3 on: October 31, 2018, 08:38:50 AM »
Lotta things can cause that, use the obd to look at any issues, such as a lazy o2 sensor,  coolant temp sensor inputs when cold, no easy answer for this

Offline gkhashem

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Re: 25 RPM drop on '91 4.9 Engine.
« Reply #4 on: November 02, 2018, 12:47:52 PM »
Barry

Dump in a bottle of Chevron Techron fuel cleaner at next fill up and see if that clears it up.
1959 Oldsmobile 98 Holiday Sports Sedan
1964 Oldsmobile 98 Town Sedan (OCA 1st)
1970 GMC C1500
1977 Oldsmobile 98 Regency Coupe
1978 Cadillac Coupe Deville (Sr Crown #959)*
1984 Oldsmobile Delta 88 Royal Brgham Coupe (OCA 1st)
1991 Oldsmobile Toronado Trofeo (OCA 1st)
1991 Cadillac Eldorado Biarritz (Sr #838)
1992 Oldsmobile 98
1996 Oldsmobile 98
*PastPres

Offline Barry M Wheeler #2189

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Re: 25 RPM drop on '91 4.9 Engine.
« Reply #5 on: November 02, 2018, 07:32:44 PM »
I shot some cleaner on it this afternoon and the idle RPM is down to 625-650 when warm. The shop manual suggests 525. The butterfly valves were pretty clean even before application of the cleaner. And by doing so dropped the RPM somewhat. Before I took it up from the garage to park it back on the street, I let it idle for about five minutes and it seemed pretty smooth.

I need to put gas in it soon, so I will dump some Techron in it when I do so.

Barry M. Wheeler #2189


1981 Cadillac Seville
1991 Cadillac Seville

Offline Dave Shepherd

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Re: 25 RPM drop on '91 4.9 Engine.
« Reply #6 on: November 02, 2018, 08:34:28 PM »
Your close to getting  it inline.

Offline TJ Hopland

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Re: 25 RPM drop on '91 4.9 Engine.
« Reply #7 on: November 05, 2018, 10:31:58 AM »
How many miles on it?   I recently picked up a non running 90 which is the 4.5 but same 'SFI' fuel system as yours and learned a lot about them in a pretty short period of time.   

These uses the external plunger dodad that acts on the throttle for idle speed like the carbs rather than the IAC valve that most EFI's used.  Its both a motor and a switch/sensor.  You can easily see it without taking anything apart.   With the engine running you can see the black plastic plunger sticking out and its got a metal slotted screw threaded into the end of it.  At idle the throttle arm should be resting on that plunger.  IF you operate the throttle as soon as you lift it off that plunger the plunger should extend a bit and then wait for the throttle to hit it.  When the throttle hits it you should see it retract till it gets to the desired speed.   IF its not extending and retracting you need to investigate why.   If its not contacting the plunger at idle you have to investigate why that is the case too. 

Service manual has a procedure to check the operation of the idle control system and set speeds.  I have a good scan tool so I was skipping around a bit but I think they explained how to do it without a scan tool.   Its done with climate control buttons and displays what mode you are in on the fuel display.  Without the scan tool I think you actually have to go probe connectors under the hood to verify a few settings and you will need some sort of tach to verify RPM's.   This procedure will tell you if you have a wear or carbon issue or defect or just things set wrong.

The other thing I have been reminded of and I think was confirmed here is these were an injector that had a lot of problems originally.  They had issues with the nozzles clogging and issues with the fuel eating the insulation off the internal coils.  I believe many were replaced under warranty but its possible yours never got done.  Original injectors were apparently Rochester.  They have a silver gray top body and metal bottoms.     It may be worth checking the ohms on em to see if they are all similar.  Easiest place to access them is there are connectors just past the ends of the valve covers.  This would be to the right as you are standing in front of the car.  THey are like 12 pin rectangular connectors and have other things in them.   Some of the injectors you can get the connectors off of but many you can't because there are things in the way so I found it easier to check them at those other connectors.   Let me know if you don't have the manual I can look up the pin numbers.  I think by the time I was checking mine I had the manual but I was too lazy to go get it so I just probed pins till I found the pairs.   IIRC a new injector was around 16 ohms.  Mine are 9-16 except for one that was 1 ohm.   I replaced the 1 ohm one with a matching one I found on epay just to get the car running but now I think I will replace the full set just to be sure I won't have issues. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Offline gkhashem

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Re: 25 RPM drop on '91 4.9 Engine.
« Reply #8 on: November 05, 2018, 11:11:30 AM »
When I originally got my 1991 with a 4.9L there was an article on the internet about the bad fuel injector design for the "new" 4.9L engine. It stated the original injector part # and the new redesigned replacements. I cannot find the article now anywhere.
But 1 injector was basically dead, three others were not spraying fuel correctly. So the car was running on only 4 good cylinders. The only symptoms were real poor gas mileage and a shimmy that felt like a tire issue when accelerating hard from 40-60 MPH. I discovered the issue when I went to change the spark plugs since the poor injectors caused really fouled spark plus.

I had all original fuel injectors (poor design ones) on a 27K mile car at the time in  2012. (the car now has 32K miles) I replaced them all with the redesigned AC Delco injectors and the problems went away.

Barry's car fits the bill as a 1st year 4.9L and one with very low mileage. So not sure if Barry has checked the injectors. Or least least pulled a few spark plugs to look at them.

I kept 2 of the old injectors I can look at the part #. The new ones are AC Delco part # 19244621. These were not cheap since my mechanic wanted the AC Delco and not a cheaper one.
1959 Oldsmobile 98 Holiday Sports Sedan
1964 Oldsmobile 98 Town Sedan (OCA 1st)
1970 GMC C1500
1977 Oldsmobile 98 Regency Coupe
1978 Cadillac Coupe Deville (Sr Crown #959)*
1984 Oldsmobile Delta 88 Royal Brgham Coupe (OCA 1st)
1991 Oldsmobile Toronado Trofeo (OCA 1st)
1991 Cadillac Eldorado Biarritz (Sr #838)
1992 Oldsmobile 98
1996 Oldsmobile 98
*PastPres

Offline Barry M Wheeler #2189

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  • Name: Barry Wheeler
Re: 25 RPM drop on '91 4.9 Engine.
« Reply #9 on: November 05, 2018, 11:26:14 AM »
It's got 22k and was the 331st Seville off the line, so it possibly has the early injectors. I do have a shop manual and it is lying on my dresser open to the appropriate page relative to idle. I saw the idle control valve the other day when I cleaned the throttle body butterfly valves. The idle valve is not stuck as when it is warmed up, the RPM drops from from about 800-900 RPM down to 625-650 at idle. The book says minimum idle speed should be 500-550 RPM.

Regular gas was low @ $2.69 this morning so I need to add my bottle of Techron and fill it up (with Premium) and see what that will do before I have the mechanic mess with it. I need to have the axles and front wheel bearings replaced on the '81 first so I"ll have a driveable "spare" before I think about replacing the injectors, but that is definitely on the schedule before a spring trip to VA to check out our great-granddaughters.
Barry M. Wheeler #2189


1981 Cadillac Seville
1991 Cadillac Seville

Offline gkhashem

  • Posts: 1118
  • CLC Number: 26576
  • Name: George K Hashem
Re: 25 RPM drop on '91 4.9 Engine.
« Reply #10 on: November 05, 2018, 11:41:23 AM »
Barry I will look at the originals I have and get you the part#. But if you have AC Delco injectors without the part number above you may have the old ones.
1959 Oldsmobile 98 Holiday Sports Sedan
1964 Oldsmobile 98 Town Sedan (OCA 1st)
1970 GMC C1500
1977 Oldsmobile 98 Regency Coupe
1978 Cadillac Coupe Deville (Sr Crown #959)*
1984 Oldsmobile Delta 88 Royal Brgham Coupe (OCA 1st)
1991 Oldsmobile Toronado Trofeo (OCA 1st)
1991 Cadillac Eldorado Biarritz (Sr #838)
1992 Oldsmobile 98
1996 Oldsmobile 98
*PastPres

 

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