Ok, this one has me puzzled and really bugged. I've owned this car for almost two years. Love the car, rides and drives great only one thing that bugs me and I can't seem to find anyone that can give me a straight up answer.

It has the 4.9L V8, FWD. Lots of power. I think its rated at 310HP. I average 23.7 mpg. Not bad for such a big car and especially the way I drive it. On the digital read out it says Premium unleaded only. How critical is this? The reason I bring it up is this engine will knock, ping and rattle the valve on anything but 92 octane. You can even hear the valves ratting under normal crusing. Sounds like a dag gone deisel engine. I have never had an engine so darn sensitive to the type of fuel you put in it.

About a month ago I saw another make model and year Caddy getting off an exit, I just happen to have my window down and it was doing the same thing. Has anyone heard anything about these engines doing this? I do notice that if you accelerate really fast it will not do it, almost as if the oil pressure is low. None of the warning gauges are on and I have never had a low oil pressure light come on. Its been to the dealerships twice for some other things I had fixed, and none of them ever said, your valve train is bad, or you have an oil supply problem or anything.

It does not lack any power, trust me, it will light them up from a dead stop and hang with any GT mustang.

I've thought about backing the timing down to see if that would help, but I was afraid it might set off a trouble code.

Any help or suggestions would be greatly appreciated, because I'm stumped .

Perfectly normal for your car. There is really nothing you can do to change it.
Yes it does need the higher octane. The engine was designed for it and cannot do without. Low octane lights easier, burn faster and hotter, but produces less heat and power.

When you reach 2500 degrees in the cylinder during combustion, you produce NOx and start to ping. This is a preignition due to the heat in the cylinder. The EGRs job is to control this, but it can't do it alone. The cooling system is a very important part. As is evey other factor in the engine and under the hood that affects the engine/cylinder heat.

That engine does not dissapate heat from the cylinders very well. You would think aluminum would do that, but it depends on the total engine design.

Yes, you could retard the timing a bit, if you have a distributor. I would have to check which year it went to Distributor less igniton. If you have the DIS, there is nothing you can do with the timing.

But even with the distributor, if you retard the timing enough to stop the ping, you will lose quite a bit of power. You won't be happy at all. And it probably won't save fuel. The power would be down enough that you would push the throttle further, and drop the mileage. It's a lose-lose situation.

The reason it does not do it on hard accelleration is fuel mixture. At hard acell, the fuel mixture becomes richer. A richer mixture burns cooler, thus dropping cylinder temp, and no pinging

Yes, there are other cars like this. You are not alone. They were designed for a purpose. Doing something else just does not work well. In some aspects, not unlike or C3s.

|UPDATED|9/27/2004 12:04:55 PM (AZT)|/UPDATED|

Thanks,

I thought you might be able to clear the air. I usually try to run 92 premium, but I've owned a lot of cars and this one is just plain finicky. I usually only think of running 92 in a high performance engine. I wouldn't classify this engine as High Performance, but its sure not lacking and certainly could be. Love the power getting onto the freeways as long as no one is in my way 80 is just 9 sec away.

Yes this one is Distrirbutor. In 95 they went to the DIS if my memory serves correctly.

I sold my 91 in 2000 with 264,000 miles on it. I still see it around town every now and then....still running.
I didn't have your problem at all, but it was hell on water pumps, and they are a pain to change. You are right, they do have balls for a big car.
C/U
Ken

Ken,

When I used to work for a Cadillac dealership in the late 80's, we had several Fleetwoods, aka "Boat Anchors" coming in with the dreaded piston knock, and hellacious pinging. These cars had the famous "HT 4100" motor in them, and as we all know, HT stands for "hook and tow".

Most of these cars were granny getters, what we used on them was "GM Top Engine Cleaner". This was one of those rev the motor, pour it in until it stalls, and let it sit overnight.

When you started it up, outside, of course, instant smokescreen that would impress an Infantryman. There was chunks of carbon that would fly out the tailpipe.
Once the idle was stable, it was up to Rt. 309 for a high speed pass.

The cars were always returned running like new. Until the next year, they would come in for the same problem. Those rascally Grandmothers.

Alan

Alan, your right. Top engine cleaner and I have removed a lot of carbon. LOTS of carbon, and make LOTS of smoke. I cleaned out a Caprice with a 454 behind the dealer one day. The smoke was so bad one of the salesmen out front called the fire department!
He could have looked first, but NNOOOOO.

There was so many problems with the HT4100 I'm amazed they actually put it in a car to start with. They were comming apart on the test stands before production started.

AfterShark, you could use Top Engine Cleaner, but I guessing with you, the problem is not carbon. And the symptom is common/normal with your engine.

I also have a 94, its a Concourse Deville but still the deville. Just had the water pump go out on a trip coming home, nursed it home, waiting for the socket to remove the guts to the water pump. As soon as its fixed I headed to the nearest dealer to trade up, I love the ride, and the engine, man it shits and gits, but I have stuck over 3000 in the last year, 135,000 miles. I have my eye on a 2002 Deville, loaded to the hilt. After talking to the local Caddy dealer, he said the 2000 and up DO NOT NEED preiumum fuel, he explained that lower compression, more computer voodoo and head design, they now can run with regulat grade unleaded, so looks like the only fix is to move up. good luck!


Dave
Mr69vett

Yes, mine has 156K on it. I had to have the fuel pump replaced on it, which cost me dearly. The pump was $194 by itself with no labor. I could have done the job myself, but had no clue why it would run and just die and have no trouble codes. Took it to the dealer.

Oh well. Wife is ribbing me to trade it in on a used mini van, because we have so many kids. I really don't want to drive one of those. Maybe I can take her Explorer and she can have the van. I really want a new Dodge Magnum with the Hemi. I don't see that happening. My wife and I need new vehicles all the way around. The cadillac has the least miles of the them all. The explorer has over 200K and my Toyota has over 170K. I just replaced the head on it and now its starting to smoke and use oil. I didn't see any damage to the cylinder walls, but It must have damaged an oil ring. Can't complain its 20 years old.

There is a good reason it did not set trouble codes. When the fuel pump quit, it stopped running. It won't set trouble codes unless it is running. There are never trouble codes from a no start.

My old Sentra went to 256,000 miles. Now the Celebrity is a rooky, it's an 89 that I got a year ago with 48,000 and now has only 63,000. The wifes mini van went with only 186,000. The present one is just broken in with 110,000.

Hi Ken
I would like to help clarify the octane rating/power issue. Gasoline is defined as chemicals in crude oil that boil between 100F and 400F(ref Marks standard handbook for mechanical engineers)
Lower octane gasoline has a lower boiling point because of the higher voletile content.This makes it harder to control ignition, thus,pre combustion cycle ignition,or
(ping) Auto ignition or what ever you want to call it.
However the lower octane gasolines have a higher energy value (BTU'S/lb) and are capable of producing more power
if you can control the burn! The first supercharged top
gas dragsters to reach 200 MPH (about 1970)acheived this
running regular gas with about 10 degrees total spark
advance.Some thing to think about!Maybe run a 160 thermostat,run the carb ritch and see how much timing
your engine will tolerate.Go faster on cheap gas?
Steve