: Northstar "Diesels" when cold



o-woody
11-05-04, 06:25 PM
I recently purchased a 1996 Deville with a 4.6 L Northstar. She had about 81K on her when I bought the car and about 83K today. ( I love the car!)

The temp has held around 194-198 degrees F. She does not "Make" oil, or loose coolant. I am averaging 19 MPG in the city and about 23 highway.

About a month ago she stalled going about 30 mph. I immediately replaced plugs and wires. Then replaced the antifreeze (with the proper antifreeze) and added the pellets. Also keeping up with oil changes.

When the motor is cold and I start driving, their is a noticeable low sounding knock or "Diesel" sound. As the car warms up, it become less noticeable. As I drive the car, it does not seem to run as "Smooth" as when I bought it.

Is this a sign of trouble? Should I worry about the cold knock?

Please advise..

Thank you,

Woody

blb
11-05-04, 08:09 PM
Ahhh......welcome to the wonderful world of the infamous GM Piston Slap. It is not as common on Northstars as other GM engines such as the LS1/LS6 family, but I have heard several Northstars with cases from very minor (just during cold starts) to severe (makes the noise all of the time to some extent). GM will tell you "it is not uncharacteristic for this engine", and "just ignore it, they all do it" but disassembled engines from severe cases tell a different story. For more information and details, see www.pistonslap.com.

BeelzeBob
11-05-04, 10:01 PM
blb is likely completely wrong. Another internet expert diagnosis. Piston slap, per se, has never been an issue with a Northstar when cold.

Likely what you are hearing is called cold carbon rap. It comes from excessive carbon build up in the combustion chamber contacting the piston causing the noise. The noise is promenent when cold due to the mechanism making the noise. The carbon buildup touches the piston at TDC and causes the piston to rock in the bore. While technically this is piston slap it is not piston slap that happens without an outside mechanism...i.e...carbon buildup. With the carbon touching the piston NO piston can avoid "slapping". The pistons in the Northstar have the correct piston pin offset and correct taper on the piston skirts to be quiet hot or cold.....unless the carbon builds up to the point that it touches the piston. Then the piston is mechanically rocked by the carbon and makes noise. The noise is much more promenent when cold due to the pistons being cold and not warmed up to the normal operating temperature and dimension. The piston clearance is set for a warmed up engine and as the piston expands considerably when hot, when cold the smaller, contracted piston has extra clearance and the "rock" caused by the carbon contact is more pronounced and thus makes more noise.

There is an easy cure for this. Quite driving it like a weeny and floor it. Do several WOT blasts thru the redline upshifts and clean the carbon out of the chambers and the noise will go away. Guaranteed. Some engines that are heavily carboned from using certain fuels and oils have required extended WOT operation and/or a combustion chamber "de-carb" using solvents such as the GM Top Engine Cleaner. IN my experience a dozen or so WOT blasts to redline when accelerating onto the expressway is plenty ample to clean out the carbon.

Search in the archives using the forum search feature in the tool bar above. Type "carbon rap" into the drop down box and read my posts and the other testimonials to the WOT procedure. The Northstar engine was designed with full throttle operation in mind. It is a high performance engine and it was assumed that customers, wanting the performance, would not hesitate to use it. Use it.

chazglenn3
11-05-04, 10:27 PM
blb is likely completely wrong. Another internet expert diagnosis.

:blasted:

o-woody
11-06-04, 10:37 AM
Thanks Bbobynski!

I have been doing WOT's, it sounds like I need to do it some more.

Also, What is "Redline" rpm for the N*?

Thanks!

Woody

chazglenn3
11-06-04, 12:03 PM
I think the redline on my STS is at 6500 RPM.

blb
11-06-04, 01:10 PM
So let's see if I understand this correctly........the PISTON SLAPS into this excessive buildup of carbon in the combustion chamber and I'm wrong in calling it PISTON SLAP? Whether the cause is excessive carbon build-up, as is the case in many Northstar motors, or excessive clearance between the piston and cylinder bore, aggrevevated by a short piston skirt as is the case in the LS1 engine family, either way, its PISTON SLAP, admittedly caused by two entirely different scenarios. Call it what you want, but either way, what you hear is the PISTON SLAPPING. ....Not a good situation in either case.

foo
11-06-04, 01:34 PM
My 99 does it.. I've done HUNDREDS of WOT's and it STILL does it.. I've sprayed TEC down the intake several times.. i just can't get it to go away.. once it warms up fully it makes no noise at all..

How do we fix this one BBOB? :))))))

blb
11-06-04, 01:47 PM
:blasted:

That's a good one! Talk about expert internet diagnosis'........I remember not too long ago bbob, gave a guy the same old "you gotta just floor it a few times" and "don't be a weenie" advice for the exact same symptoms on a Northstar, and a piston rod let loose and put a hole through the side of the guy's block. Oil and aluminum all over the street........yep....now it was REALLY fixed! LOL.....

eldorado1
11-06-04, 01:55 PM
I think we can all agree, that you can't diagnose an engine problem from 500 miles away... Sometimes you just gotta hear the knock, or whine, or whatever for yourself. I have to think that blb is basically right, it's just a matter of semantics.

chazglenn3
11-06-04, 05:46 PM
My 99 does it.. I've done HUNDREDS of WOT's and it STILL does it.. I've sprayed TEC down the intake several times.. i just can't get it to go away.. once it warms up fully it makes no noise at all..

How do we fix this one BBOB? :))))))

Head over to your local Napa and buy a can of Sea Foam. You won't believe the cloud of white smoke your car will make as it cooks that crap out of the engine. I fogged out the whole neighborhood when I used it in my 87 Supra.

96 STS
11-06-04, 07:12 PM
Yeah, what about this SeaFoam stuff? Has anyone here used it in their Caddy. I've been thinking about dropping some of this into mine to clean it out, but I'd rather have other n* testamonials (sp?).

So....any of you used this in your n*? If so, what did you think?

96 STS

chazglenn3
11-06-04, 08:35 PM
Well, I have used it in my Supra and the wifes Explorer, but haven't had a need to put it in the Northstar...the wide open throttle blasts seem to be working for me. I was getting clouds of smoke after every blast until the last 4 or 5 that have been clean.

dkozloski
11-06-04, 09:03 PM
Warm the car up, get it turning at about 2000RPM, and spray plain water generously from a soap bottle or the like in the air intake with the filter removed. If the engine starts to miss bad enough to stall, slow the spraying. Keep up the spraying until the black exhaust cleans up. There won't be enough water going in to hydraulic lock it or do any other kind of damage but the thermal shock of the water droplets will break up and remove a large part of the carbon buildup. There used to be a nationwide franchise for a system that cleaned the engine through the sparkplug holes using a blast of walnut shells. The water works at least as well. I have been using this procedure for at least 50 years and have never damaged anything by doing it. FWIW, they clean $1,000,000 jet engines with a water washdown.

eldorado1
11-06-04, 10:11 PM
Warm the car up, get it turning at about 2000RPM, and spray plain water generously from a soap bottle or the like in the air intake with the filter removed.

I was just going to suggest that! If your engine is really crudded up, you'll be surprised how smooth it'll idle after you're all done. IMHO, about a quart of water is what you'll want to run through it. Seafoam works as well, but I don't think there's enough of that stuff in a single bottle - if you do go that route, get 2. But then, distilled water is free (or close to it compared to $6+ a bottle)

BeelzeBob
11-07-04, 01:11 AM
That's a good one! Talk about expert internet diagnosis'........I remember not too long ago bbob, gave a guy the same old "you gotta just floor it a few times" and "don't be a weenie" advice for the exact same symptoms on a Northstar, and a piston rod let loose and put a hole through the side of the guy's block. Oil and aluminum all over the street........yep....now it was REALLY fixed! LOL.....


Well, it quit making noise, didn't it....LOL I really shouldn't joke about it. I did feel bad that that happened. It is hard to diagnose all these problems and be right 100% of the time with limited information and all. I do hate it that he broke the motor because of my information.

For the one that broke when he floored it there have been several hundred others that were fixed by doing a few WOTS. Besides, if you recall, I also suggested that he first cut open the oil filter and see if there was any debris in it from something serious coming apart. He bypassed that step and took it onto the freeway in second gear. If he had taken the time to follow ALL the advice he would have caught the problem before he blew it up.

I suppose piston slap is piston slap. Your comment originally related it to generic cold piston slap due to excessive clearance or something and provided no method of correcting the problem. The carbon build up and the cold carbon rap is a specific situation that occurs on the Northstar and there is a solution or remedy to it which you didn't mention or touch on in your answer. The fact is that the noise caused by carbon build up is really a different mechanism that is not what normal cold piston slap is...so I would only give you half a point for having the half the correct answer....LOL LOL

BeelzeBob
11-07-04, 01:21 AM
My 99 does it.. I've done HUNDREDS of WOT's and it STILL does it.. I've sprayed TEC down the intake several times.. i just can't get it to go away.. once it warms up fully it makes no noise at all..

How do we fix this one BBOB? :))))))


It makes the noise all the time or just cold???

If it does it just cold and the noise goes away when hot it might be a touch of piston slap that is not being caused by carbon build up. It can happen. There is always a ragged edge of having just enough piston pin offset and piston clearance for good operation hot and then getting a little piston noise when cold until the pistons warm up. Typically, this has not been an issue on the Northstar except when there is excessive carbon build up. But it still could happen I guess if you are keeping the carbon cleaned out for sure.

Cold piston slap is certainly not desireable but it is not going to hurt anything at all. There is a tradeoff with tight piston clearances and friction and HP. You want enough clearance so that there are no hot scuff issues and the motor has low friction and makes good power and on the other hand too much clearance will cause a little noise cold. The short, lightweight, low friction piston designs leave little to no room for error here. Just an extra micron or two in clearance can cause a little noise cold. Forget about it and drive it I guess.... It really isn't going to hurt anything at all.

growe3
11-07-04, 11:45 AM
One way to check if you have excessive carbon buildup is to do a few WOT in a safe way. Usually a freeway on-ramp provides this opportunity.

With no one in front of you floor it from about 20 MPH. Let the transmission shift when it wants, that will be about 6,500 RPM. Conditions permitting take it to 80 MPH.

When you exit the freeway use the transmission to slow you down. Just drop it in to third till you stop. This will help excerise the rings to keep them free. It will help both the compression & oil rings seal better, for less oil usage, and keep the piston centered.

Do this a number of times. When you do not see any smoke/dust come out of the exhaust, when accelerating, the excess carbon is gone.

If you do at least one or two WOT's a week it will not build up again. Make a habit of using the transmission to slow you down, just as a stick shift is used. I noramlly only use 3rd for this though. If stuck behind slow traffic on a long dowgrade, I might drop down to 2nd to save on braking actions.

None of the above is abuse to your engine or transmission. All of the above in kinda fun though.

-George

foo
11-08-04, 09:20 AM
It makes the noise all the time or just cold???

If it does it just cold and the noise goes away when hot it might be a touch of piston slap that is not being caused by carbon build up. It can happen. There is always a ragged edge of having just enough piston pin offset and piston clearance for good operation hot and then getting a little piston noise when cold until the pistons warm up. Typically, this has not been an issue on the Northstar except when there is excessive carbon build up. But it still could happen I guess if you are keeping the carbon cleaned out for sure.

Cold piston slap is certainly not desireable but it is not going to hurt anything at all. There is a tradeoff with tight piston clearances and friction and HP. You want enough clearance so that there are no hot scuff issues and the motor has low friction and makes good power and on the other hand too much clearance will cause a little noise cold. The short, lightweight, low friction piston designs leave little to no room for error here. Just an extra micron or two in clearance can cause a little noise cold. Forget about it and drive it I guess.... It really isn't going to hurt anything at all.


Ya.. I blast on it all the time.. not always to 6000+ rpm but i don't accelerate slow.
I'll try doing 10 WOT's on the highway from 40-85 (2nd gear) and letting 2nd gear slow meback down to 40 and then ill park it and see if it does it the next morning.
There is SOOOOOO Much gunk built up in my intake it's scary.. every time I use the top engine cleaner down the throttle body the car stinks for weeks and it's a horribly nasty smell and i get p0420 errors afterwords also :/