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Low RPM intermittent missfire?


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LOW RPM Missfire driving me KRAZY! :blink:

Now what? My 96 Eldorado (60,000 miles) still has an intermittent misfire on #1 cylinder. The darn thing might run fine for weeks at a time and never miss a lick then when you least expect it, it starts missing again. Sometimes it happens when the engine is cold, other times it happens after it's warmed up. When it sets a code it is always #1 cylinder misfire.

One thing you can bet on, it darn sure will not misfire when you take it to a tech to be checked out! 3 trips to the shop, ran great all 3 times.

So far I have Checked all connections wires plugs etc., installed New AC plugs properly gaped, New Factory plug wires, replaced the coil pack with factory A/C Delco replacement. Compression test is good, plugs are burning good, no coolant loss, average oil consumption for a Northstar (1 qt 1500-2000 miles) and the fuel milage it great.

Engine idles fine but misses sometimes on acceleration or when pulling a slight grade. The misfire usually only occurs between 1500 to 3000 RPM otherwise, the engine runs fine when you punch it and very rarely misses at higher RPM. There have been a couple of times that the engine sounded like it backfired just as the misfire started. (Fuel injector or EGR valve maybe?) I have preformed all of the different suggested test that I have seen on the Cadillac forum with no negative results and now am about to resort to parts changing as a last ditch effort. Love the car but, Next comes the 16 lb sledge or maybe a trip to the crusher. LOL

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This is an odd one, and you replaced everything that I might have suspected (plugs, wires, coil pack, and compression is good).

I am sitting here wondering if a dirty injector could cause this or if the injector triggering wiring could cause this, I you are going on a trip you might try to run a small bottle of Techron through the system, DON'T let it sit, run it through your system VERY fast like on a trip where the cleaner is out of the tank as fast as possible. AND add it ONLY to a full tank.

I would check #1's injector connection for corrosion and carefully separate the ignition wires from all of the injector wiring. If the ignition/injector wires must be near each other never let them run side by side for any distance at all. When you pull the plug wire on the #1 plug do you have any oil on the boot? If the cam cover spark plug seal is leaking it could provide a path for misfire.

How often do you do WOT's? If you consider going to the crusher, send me an PM with your phone number, I'll take it off your hands and won't even charge you for transporting it, it will make a nice partner for my 96 Deville ;) Hope this helps.

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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Same symptoms I had on my 94 Concours. It had me frustrated for months. Turned out to be the coil for the problem cylinders, I replaced it and all my troubles went away. Take the #1 coil off and check the contacts, clean them and put it back together. I know you said you replaced the coil pack but check that coil connection. You might double check the plug wire route for the #1 cylinder. Even a new plug wire can arc if it is touching a piece of metal.

I have been there and I feel for you, keep up the faith.

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Hi Krazy,

Sorry to hear of your misfire problem. The fact that it backfires just before the miss develops leads me to think of a sticking valve or a tappet (lifter) that is pumping up too much. Either of these problems could be caused by dirty oil, engine running too cold or an actual mechanical problem.

It also could be the injector as Scotty saya. If it (the injector) was running too lean, it could cause a "lean pop" that could sound like a backfire. I would try Scotty's suggestion of running a dose of Techtron thru a tankful and see if that helps. If it does, great.

If not, then I would try changing the oil and replacing one quart of oil with a quart of Rislone. I wouldn't leave the Rislone treated oil in for more that a 1000 miles. I don't normally recommend oil additives for modern engines, but I would try this before I did any major engine work. As Scotty suggested with the Techtron, I would take the car on a trip of a couple of hundred miles or more just after adding the Rislone.

Some people might object to the Rislone treatment. If Bbobinski objects, then I wouldn't do it. Otherwise it would be worth a shot.

I have seen Rislone free sticking valves and cure squeaking rocker arms on engines that have been neglected in the past. So that is why I suggested it here.

I am wondering what kind of usage does your Eldo get? Is it mainly short tripped? If it gets less than 10 miles at a time or less, then it isn't getting hot enough to evaporate the combustion contaminates that form in the oil. this could be a cause of the sticking valve or lifter problem, if in fact that is what the problem is. The soloution to that is to Drive More. :P 60,000 miles is not much for a 9 year old car.

Before you take that 16 lb. sledge to it, give me a call, I will beat Scottie's offer, by offering you a even $100.00 for it. ;)

Anyway, good luck finding you miss.

Take Care,


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I just went through a misfire issue on my 96 STS. I changed wires, plugs and two ignition coils and still had a misfire (P0300) - what a ball buster. I had to take it to the dealer. I'm $140 lighter today, but my car is fixed. So I asked the mechanic what was wrong - he told me I had one defective spark plug and wire. I was bullxxxx, :angry: I install a defective spark plug and wire (brand new parts). My car runs great now (knock on wood).

Check the spark plug wire resistance and the spark plug with an ohm meter. The spark plug should read open (I have an analog meter) from the spark plug tip (where the spark plug wire connects to) to the electrode.

My spark plug read 7000 ohms.

I spoke with the mechanic, who told me he sees this alot.

Good luck,

Chuck :)

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Check the condition of the wires by spraying water mist on the wires at night while the engine is running.

If you see spark leakage then the wires aren't good.

The other thing that may cause this problem is your fuel pressure regulator, disconnect the vacum hose from it and check if there is fuel in the vacum hose. If the diaphram of the regulator is burst then you will see fuel leaking into the vacum hose, and as you accelerate more fuel will get in to the air intake and messes up air/ fuel ratio and may cause some operating problems.

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Thanks guys! I have pretty much tried all of these suggestions. Misting, checked fuel regulator, oil is changes regularly, fuel filter, connections etc. I think I may have found the problem. I took one of my old plug wires and just stuck it on #1 and so far it hasn't missed a lick so hopefully the new wire was defective. BUT, that really doesn't mean anything yet because it's been known to fun fine for long periods of time and then start missing again so I guess we'll wait and see. Todays project is replacing the water pump on my girlfriends Jaguar XJS-TWR -V-12. Talk about cramped spaces!!

Thanks again & I'll keep you posted.


PS, It get's WOT at least one time almost every time I drive it. Love the power and at WOT it never missed and my son can't stand it when I dust the doors off of his 5.0 Mustang GT. LOL

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