Which engine ? Northstar ? ...... and which model level - ESC or ETC ?

 

It is the 4.9. Pretty sure it's the ESC model.

 

Gas is unlikely. I'd start with the basics, plugs, wires, distributor cap & rotor.

 

Just to be clear, it doesn't do this all the time. This would make me believe it is not a broken part. Wouldn't this symptom come on gradually if it was just normal wear and tear on the plugs, wires, etc?

Not trying to be difficult, but this is more like a switch is flipped and it starts running rough. Usually, I start the car in the morning, it is fine until I put it in gear, then it will start missing. Misses until a back out of the driveway, then usually after I am in second gear, it smooths on out. Then, at highway speed, sometimes it will get the entire 20 miles to work just fine, other times it will seem like it is missing sometimes.

I didn't know for this engine if 75,000 miles was a reasonable time to expect plugs to need changing.

 

91 Fleetwood had the same problem - fuel pump. Fuel pump failure ultimately was due to plastic baffles in the fuel tank cracking/crumbling and gumming up the pump filter.

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Sure sounds like plugs and wires. Easiest fix.

 

ok, new plugs and wires sounds like the plan for this weekend.

 

Don't forget the cap & rotor.

 

More info....

I was driving home last night and it was running bad, really BAD. Coming into Kentucky from Ohio in Cincinnati, there is a mile long steep uphill climb right after you come over the river. I floored it halfway up the hill, let the car shift at 5100 rpm.

The car ran flawlessly after that on the highway, and also as I ran a couple of errands in my neighborhood. Perfect.

Started it up this morning...missing a little. By the time I got to about the 15th mile of my 20 mile trip to work, I was beginning to wonder if I was even going to make it to work. Stumbling really bad.

Still sounds like plugs, etc? Probably. My biggest worry is that, once before, the "ignition module" went out on the car and left me stranded on the side of the road. Not really the same symptoms...when the module went out, it just stumbled for about a hundred yards and then went totally dead. Hope that isn't contributing to this problem.

Crossing my fingers that I make it to Saturday!

thanks for all the replies. Just ordered from RockAuto...

Part Number Part Type Price EA Core EA Quantity Total
1993 CADILLAC ELDORADO 4.9L V8
ACDELCO D338X Distributor Cap $ 13.97 $ 0.00 1 $ 13.97
ACDELCO D468 Distributor Rotor $ 8.48 $ 0.00 1 $ 8.48
ACDELCO 628B Spark Plug Wire Set $ 41.79 $ 0.00 1 $ 41.79
NGK 3403 Spark Plug $ 2.21 $ 0.00 8 $ 17.68
Shipping Ground $ 8.99
Order Total $ 90.91

 

Put on the new cap, rotor, wires and plugs tonight. Driving it home, the car is doing exactly the same thing it was doing before. No improvement. Missing so bad it feels like the motor is WAAAYY out of balance.

All the old plugs were white powdery dry... I mean really white...

 

Have you checked the fuel pressure?

 

:yeah: Fuel pressure is easy and quick to test. If that checks out, I wouldn't rule out an injector not firing on time. Either due to a poor connection or simply a bad injector, which these were known for. I believe there is a way in diagnostics to turn each injector on and off. Maybe someone with more intimate knowledge of this year can chime in and give the procedure.

 

I'll give that a shot next. I'm assuming there's someplace I can connect to under the hood?

 

On my TBI car there is a valve on the fuel line. I think the port injection like yours have the test valve on the fuel rail.

 

Yeah, it'll look just like a tire valve. Just screw the pressure tester onto it.

 

Excellent photo! Just had this cover off routing the spark plug wires Monday evening, so I'm very familiar...

Funny thing about the way the car is running, every once in a while, it will surge a little, pick up a couple of hundred rpm, then smooth on out and run fine for a while. Then, especially if I've been off the throttle for several seconds coasting in traffic or down a hill, it will go back to missing again. In general though, in traffic on the highway, it just doesn't have any power and you can feel it missing, especially below 2000 rpm.

I think while I'm at it, I'm going to change the fuel filter and maybe run some injector cleaner through the tank.

Are there any particular test to do while checking fuel pressure, or just see what it is at idle? What should it be at idle?

 

Your fuel pressure should be about 40 - 50 psi. Check it at key on and again at idle. Also at throttle up. It should remain pretty steady. If you have a gauge with a long enough hose, tape it to your windshield and take it for a ride.

In general though, in traffic on the highway, it just doesn't have any power and you can feel it missing, especially below 2000 rpm.

That could also be a clogged CAT.

 

Here's what's been going on with this...

I spent a couple of weeks trying to find a fuel pressure test kit that I thought I had, but could not find it. Then another few days agonizing over whether I should spend the $25 for a new kit. Finally, yesterday I went to AutoZone and "rented" one. Cost $150 for the deposit!

Here are the tests I did...

Fuel pressure at:

key on - 38 psi
idle - 35 psi
revving to 2000 rpm - jumps to 40 psi then drops to 32 when letting off gas, then stabilizes back at 35 psi
With vacuum unplugged from pressure regulator - 44 psi

Also, I checked to see if pressure bled off after the pump shut off. No pressure bleed off...holds steady at 38 psi after turning the key off.

Made a line extension so I could run the gauge back to the cockpit (held gauge in my hand through the window) and took it for a drive. Car was in "miss mode" when I took it out.

Drove it out on the street and pressure was 38-40 when on the gas. Drops to 32 when off the gas coasting. I went up a hill and fired down on the gas. No power for about the first 100 yards, then it was like a kick in the pants. The cylinder(s) that was not firing kicked in, and it was like I had another 50 HP under the hood. This is the behavior it has always exhibited. If running good, it seems that high vacuum conditions (coasting in traffic or down a long hill) will trigger the miss. Then it will take it a while to clear itself back out, but if I shower down on the gas, most times it will clear itself up and run right for a while. Anyway, no unusual fluctuations with the pressure, generally stayed between 35-40.

While I was under the hood, I checked that there were no drips of gas coming out of the vacuum line when I had the FPR unplugged. Also, did the screwdriver stethoscope trick on each injector. I could hear the "click-click-click" plainly on all but #4 cyl. That one, I can still hear the injector firing, but the click is not quite as sharp as all the others.

So I parked the car and fiddled around with some other stuff at the garage for a couple of hours. Got in to drive home, and the miss was back. Pretty bad. It drove bad the entire 5 miles home.

This morning, I decided to try something. I unplugged the FPR vacuum and plugged the hose. I wanted to see if the fuel system worked better at 44 unregulated psi. Started my commute to work. Car still driving like crap. After a couple of minutes, car started running fine, and ran just fine the entire 20 miles to work.

So....starting to think that the fuel pump is getting tired, and that it is just on the border where some of the injectors are happy, and some are not. I would have thought though, that the car would have run correctly immediately after unplugging the FPR instead of taking a couple of miles to smooth out.

Anyways, that's where I am and I appreciate anyone's experienced input into my situation with the car. It's sounding like there's probably a fuel pump R & R in my future. I am thinking about using the "cut through the hump under the back seat" method instead of dropping the tank.

 

Sounds like your fuel pressure is a tad below factory spec's, but I'm not sure it is low enough to cause your problem. When the fuel pump got weak on my '92, it would run fine until I went WOT. Then the fuel pressure would fall off and the car would fall flat on it's face. It would stay running, but with no acceleration.

Also, did the screwdriver stethoscope trick on each injector. I could hear the "click-click-click" plainly on all but #4 cyl. That one, I can still hear the injector firing, but the click is not quite as sharp as all the others.

Why not take a trip to the Men''s Mall and pick up an injector or two. Swap out that #4 just to rule it out.

 

I'm thinking about just running it unregulated for a few days (since 44 psi is the highest it seems to ever go) and see what happens.

What would be wrong with that (except probably just living on borrowed time)?

 

Have you checked to see if the FPR is leaking (internally)?

 

I thought that was what this "While I was under the hood, I checked that there were no drips of gas coming out of the vacuum line when I had the FPR unplugged." was...

Is there something different I should do?

 

Well, my optimism has been short lived. Just got home from work. Car stumbled/bumbled most of the way home. Not quite as bad as with the FRP hooked up, but bad enough. I am going to wait a couple of days for any more responses. If I don't hear anything to the contrary, I guess I am just going to get these 8 injectors and replace them all. No sense in having to take all that plumbing loose more than once....

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I just looked at those injectors on eBay again. They were $150 when I looked this morning. Now they're listed for 82.50. They're mine now! $90.42 with shipping.

 

Try filling the tank up and see if the problem goes away , to rule out tank baffle.

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Filled it to the brim yesterday. Still has same problems. thx for the idea though...

Going to try to get the injectors installed this weekend. Still running with the FPR vacuum disconnected. Problem seems to be getting a little worse. I swear the car is only running on 5 cylinders sometimes... Then it'll seem like it picks up a cylinder, then another, sometimes it even runs good for a while. You can really tell when another cylinder starts firing. It's almost like flipping a switch. Then, before long, it will start missing again, usually after having been off-throttle.

Sure hope these injectors fix it. Otherwise, we're almost back to square 1 again. Also replacing the noisy power steering pump this weekend since I have to unbolt it anyway...

 

Did you ever deduce whether or not individual injectors can be enabled/disabled via diagnostics?

 

OK, I think I found how to do this with the PCM override from another post on here. That post talks about ECM and my car's codes say PCM, but they should be the same, I would think.

 

I was having the same problem on my 1991 4.9 - I did all those things above, and same problem. Here is what finally worked for me, read my post about it below:

Re: Mysterious missing issue
Alright everyone, here is the answer to my mystery issue…
Wait for it…
FUEL INJECTORS !!!
Since I had virtually ruled out everything else, I figured this had to be it – this was my last hope.
And my professional mechanic missed it!! I don’t think your average shop is equipped to adequately test them, and they figure if the signal is getting there, and gas is flowing, then that’s it – no problem. Couldn’t be more wrong! One of them was actually missing the tip, and so was just dumping fuel instead of atomizing it, and that was the cylinder that would keep acting dead – because it was getting flooded out! Credit to whoever posted about Mr. Injector when I was searching the forum for posts on fuel injectors – I sent all 8 injectors to him to be tested & refurbished, and guess what? 6 of them would not pass the electrical test (2 had been replaced by the previous owner – the other 6 were original), and that’s not something that can be fixed/refurbed. As he explained, they will still squirt gas, but will cause performance issues since they will not properly respond to signals from the computer to operate differently according to current conditions and demand. He did not charge me for the test, and immediately referred me to Fuel Injector Connection – another company specializing in refurbishing fuel injectors, who had a set of 8 refurbed Bosch type two injectors ready to ship – these are the recommended replacements, as the originals have a reputation for having issues. He referred me because he did not have any available at the moment, and FIC did. Very nice of him, and he did not charge me anything for the test. And wait for the best part… all 8 from FIC were only $129.00 plus $20 express shipping!! Unbelievable, when new ones are at least $80 and up each! Anyway, I would encourage anyone having performance issues, to rule out fuel injectors before going down too many other rabbit holes – especially if your injectors are original. According to Mr Injector, the 1990 to 1995 4.5 and 4.9 Cadillac engines had injectors by Rochester Multec installed, and these are listed on his “Problem Injector” page because they had a tendency to short out and cause misfires.
To check either of these places out, they both have websites – www.mrinjector.us and www.fuelinjectorconnection.com . You should go to Mr Injectors site just to see all the equipment he has for refurbing and testing fuel injectors – very impressive – and he only charges $17 per to fully refurb and certify each injector. For my money though, I would replace with the refurbed Bosch injectors – mine came with a 3 year warrantee. Now she is running purrrrfect – what a relief!