Is driving a cold engine bad? - Off-Road Forums & Discussion Groups
Jeep-Short Wheelbase All discussion of short wheelbase Jeeps: CJ, TJ, YJ and JK

LinkBack Thread Tools Display Modes
post #1 of (permalink) Old 02-21-2002, 10:43 AM Thread Starter
Join Date: Jul 2000
Location: New York
Posts: 308
Thanks: 0
Thanked 0 Times in 0 Posts
Is driving a cold engine bad?

I have an '87 Yj with a 258 and Howell TBI system. According to Howell, one of the perks is being able to drive the Jeep without having to wait for it to warm up. Anyway, I was just curious, is there any potential or actual harm that can be done to the engine by driving it before it warms up? From my experience with this Jeep, if I let it warm up, it won't shake as much at idle, but if i don't and I am idling in traffic the steering wheel and shifter will be vibrating. Any ideas are appreciated.



1987 YJ 258 I6, NV 3550, TFI Upgrade, MSD 6A, TFX Exhaust, Smitty Sure Steps, Howell Fuel Injection 2.5" Rubicon Express Extreme Duty lift, 2" Body lift 1999 Cherokee Sport, Stock
donnyg is offline  
Sponsored Links
post #2 of (permalink) Old 02-21-2002, 11:01 AM
Posts: n/a
Re: Is driving a cold engine bad?

The quickest way to warm an engine up is to drive it. The key here is moderation and common sense. You don't want to stomp on the gas and hit 80+ with the engine temp below 100.

Modern fuel injection systems have really helped the cold manors of engines. On yours (and ours), the water circulates through the intake manifold. This helps warm it up quicker. The engine is designed to operate best at a certain temp. Below that temp, effeciency suffers. I don't belive the Howell is a close loop system (please correct me if I'm wrong), hence, it does not use the O2 sensor. Even on standard FI systems, the O2 sensor is ignored until the engine gets up to temp.

So, the only real issue is the mechanics of the engine. When it's cold, things are a little looser. The tollerances are designed for a certain operating temp, so until you get there, it's going to be loose (God, I hope I'm useing "loose" correctly). This is not really a problem as long as you don't abuse the engine. In other words, keep the RPM's and the loading down. Avoid the fwy untill you have gone about a mile or two.'s ok to start it up, wait a few of seconds to get the fluids moving, and then go....nice and easy. The engine will warm up quicker this way than just letting it idle for 5 minutes.

84CJ7, 3"lift, 32"BFG, 4.10's, R/ARB, F/Detroit, Onboard Air....89 Cherokee - New Project

They say we learn by our mistakes.....I guess thats why we are all so smart.
post #3 of (permalink) Old 02-21-2002, 11:04 AM
Join Date: Jun 2001
Posts: 1,628
Thanks: 0
Thanked 0 Times in 0 Posts
Re: Is driving a cold engine bad?

I don't know how significant the effect is, but technically, it would accelerate wear. The engine contracts when it gets cold, reducing bearing clearence and making the cylinders slightly smaller. A 258 designed to operate at about 195-225 degrees. The bearings and piston/bore size are designed with that range in mind. Any cooler, and things fit a lot tighter. (Running a thermostat lower than 195 is also bad for it) The engine also dosn't run near as well at lower temperatures (as you've experienced with the vibration when cold).
Personally, I don't wait on mine to reach 200 degrees before I drive, but I do let it run for 2-3 minutes before taking off.

-Derek Schwab
'97 Sidekick Sport
'83 CJ-7
dschwab9 is offline  
Sponsored Links
post #4 of (permalink) Old 02-21-2002, 11:13 AM
Jim_Lou's Avatar
Join Date: Dec 2001
Location: Belleville, Illinois
Posts: 11,502
Thanks: 15
Thanked 22 Times in 17 Posts
Re: Is driving a cold engine bad?

DDawg has it right. I have NEVER heard of a manufacturer recommending warming up at idle. Most wear occurs in the first few seconds after startup. Most of the rest occurs while the engine is coming up to operating temp. Gentle driving gets you through that period quicker. Quicker warmup also means less gasoline contaminating the oil.

I might have been born at night, but it wasn't LAST night.
Jim_Lou is offline  
post #5 of (permalink) Old 02-21-2002, 01:21 PM
Posts: n/a
Re: Is driving a cold engine bad?

Being that I was raised on a farm where there is a lot of mechanized equipment, I was always taught to allow the engine to come up to operating temp before putting a load on it. We did this with everything from the family 'burban to the gas operated generators. So still to this day I allow my vehicle about a 10 minute warm-up period in the winter and about 2-3 minutes in the summer.

Something else to think about here is what is the ambient temp? What type of oil are you using (synthetic)? Is the vehicle sitting out side or in a barn? Is it a long distance to where you are going (will the engine get a chance to get up to temp?)? Etc... In other words, there are a few more factors than just how long to idle an engine. .02 worth[img]/wwwthreads_images/icons/smile.gif[/img]

I am a bomb technician, if you see me running, try to keep up [img]/wwwthreads_images/icons/shocked.gif[/img]
'85 Cj7,360,TH400,D300,Pro-jection,D44's,M/T's 35's,8274
post #6 of (permalink) Old 02-21-2002, 07:40 PM
Join Date: Sep 1999
Posts: 5,383
Thanks: 0
Thanked 1 Time in 1 Post
Re: Is driving a cold engine bad?

I have the hardest time drilling that into the heads of customers and new employees in the engine shop...

Everyone wants to machine parts while they are cold with no load on them, and expect them to operate correctly when they are warm with a load...

When you bore and hone a cylinder on a stripped block, you get a nice round cylinder while the engine is cold with no head bolted on.
Cast iron is SOFT, and distorts enough you can actually see it deflecting/ distorting under load.

A. If you heat the block, the bore expands, and the head bolt holes actually move farther apart.

B. If you have a head bolted on the cylinder, the cylinder top will distort.

C. Heat that block and head up, and you have even more distortion.

D. Heat that block and head up a different rates (like when an engine is first started) and you will have shifting as well as two rates of distortion going on....

E. Heat will affect the top of the deck to cylinder top connection at a different rate than the cylinder walls them selves.
The deck will transfer heat away from the top of the cylinder faster in the beginning, the stress of the head bolts will move that cylinder 'mouth' around in strange shapes, and if you have connected cylinder walls between cylinders (Siamese cylinders), you can add oval cylinders to your problems...

F. Now add in the expansion of the piston, and piston rings...
The piston is aluminum, and will expand at roughly twice the rate and twice as much as cast iron.
The piston also has the distinction of having several different amounts of mass in it's design.
The bosses around the wrist pin will expand more because there is more material, but they will expand slower because it takes extra mass longer to heat up.
May makers cut the tops of the pistons (makes the head thinner in places) or drill on the bottom sides of the piston heads to 'balance' the pistons (again, making the head thinner in places).

Piston rings take a REAL a$$ whoopin' through all of this! Their piston grooves expand and contract, the piston diameter expands and contracts, and it expands in odd shapes, the cylinder expands and contracts in odd shapes around the ring, and the ring it's self expands and contracts in the middle of all of this chaos...
The top of the cylinder is where your compression pressure is highest, so any small gap in the piston ring to cylinder wall fit will be a MAJOR leak.

When we do an engine here, we always heat the block to appx. 180 Degrees, Use heated lubrication for the hone, and use a torque plate to simulate the load of the heads, and have the main caps on and torqued to simulate crankshaft main cap distortion.
This way we get a nice round cylinder when the block is hot and has heads bolted on.

On some V-8 types, we even bolt the two torque plates together to simulate the intake manifold and the way the intake will cause the heads to move when the block heats up.
On top of all of this, you have to realize the engine actually grows taller, and in the case of the V-8's, wider because the two banks of 4 cylinders each of the 'V' are growing 'UP' in different directions
Then figure the gallery end plates that hold those cylinder banks together are pulling in on the cylinder banks as they try and grow 'UP' .....
The cylinder banks are literally being pulled in a 'U' shape as they heat up and expand upward.
It takes as long as 1 minute after start up to get a reliable oil supply to the top end of older engines.
Bearings can go as long as 30 seconds without oil on startup.
All of this is slow death to your engine.
By the estimations of SAE, ASE, EAC, GM, FoMoCo, Ect. 75% to 90% of all wear on engine parts occurs at cold start ups....

The bearings rely on an oil film being held in place by OIL PRESSURE. There is no oil pressure at startup.
You are grinding metal on metal at the main bearings to crank, crank to connecting rod, connecting rod to piston, cam to block, cam lobe to lifter, ect...

Pretty destructive stuff... Dry starts are the absolute worst thing you can do to your engine, and on most stock vehicles, unavoidable.
If you have a turbo charger, the turbo is the first thing to stop getting oil (at the top end of the oiling system) and last thing to get oil on start up (again, at the tail end of the oiling system).
Some turbos will go for a minute or more before they get a reliable oil supply, and will spin VERY HOT for up to five minutes after the engine is shut down with no oil...
So, lets recap...
Cylinder tops that are star shaped and expand and contract spreading the head bolts at angles, heads that expand at a different rate, and crawl around on the block deck surfaces like a snake in a frying pan.
A Block that is growing taller and shorter, and in the case of a 'V' cylinder configuration, cylinder banks that are 'U' shaped.

Pistons that expand and contract barrel shaped, and at two to three times the size and rate of the cylinders.

Rings that have to try and seal the entire mess up enough to keep the engine not only running, but make horsepower and torque.

And do all of this with little or no oil protection.

And people still wonder if they should allow the temperature to stabilize before driving ANY engine...?
I'm always stunned that any engine runs at all!!!

So many cats, so few recipes...
Junk Yard Genius is offline  
post #7 of (permalink) Old 02-21-2002, 08:37 PM
Posts: n/a
Re: Is driving a cold engine bad?

Any NPR fans may reacall "Click And Clack" The automative experts from their weekend Q&A show Car Talk, being posed the same question, their response, any warming beyond arpoximately one minute is a waste of time, as the long winded gentlemen whom also replied alluded to, simply to get the engines lubrication going. personally beyond that if driven like a same person the engine will come to temp quicker under load, so go for it. I will admitt to having considered purchasing a remote car starter simply because Michigan winter mornings can really suck and a warm car would be nice but your and my 258 engine are some of the most forgiving engines ever built. give her a little abuse.

post #8 of (permalink) Old 02-22-2002, 02:34 AM
Join Date: Oct 2001
Posts: 1,720
Thanks: 0
Thanked 0 Times in 0 Posts
Re: Is driving a cold engine bad?

Cold engines are a big concern in Alaska, where we often get sub-zero temps. Back when I was actively involved in the mechanical industry I used to help set trucks (mostly Ford and Chevy Diesels) up for work on the North Slope. We put in oil pan heaters, a lower radiator hose heater, either a battery blanket or better still a trickle charger, and often a 1000watt cab heater for driver comfort. If the rig was an auto, sometimes a transmission heater too. All were 110volt so you could plug in the vehicle when it wasn't running. Some outfits simply never shut their rigs off in the winter, or parked them in a garage. The upshoot of all the heaters was to warm lubricating liquids up so they would flow rather than be mollasses. Many stock vehicles in Alaska are sold with "block heaters" that warm up the antifreeze with the idea it would keep the block from breaking in really cold temps. They help, but if you can only have one heater on an engine it needs to be a oilpan heater. The more liquid the oil is, the faster it will get to all the parts. Myself, I run full synthetic oil and have a oil pan heater. That way I have oil pressure as fast (maybe even faster!) during the witner as I do during the summer. Doesn't really answer your question, but does show some of why good oil circulation is important.

It's all a state of mind, and if you don't mention the state of my mind, I'll be happy to overlook yours!
cj5_pilot is offline  
post #9 of (permalink) Old 02-22-2002, 08:13 AM
Posts: n/a
Re: Is driving a cold engine bad?

With all the expansion /contraction characteristics considered , would you recommend torqueing the exhaust manifold bolts cold or hot ?

post #10 of (permalink) Old 02-22-2002, 11:27 AM
Join Date: Sep 1999
Posts: 5,383
Thanks: 0
Thanked 1 Time in 1 Post
Re: Is driving a cold engine bad?

"Long Winded"?
I thought a complete answer is what was wanted, not just a 'Yes or No' answer, Sorry.
As for the torquing of exhaust manifold bolts...

Was that answer short enough for the new guy?
You should torque your exhaust manifold bolts three times when you first start the engine, or use bolt locks.
1st. time, cold, when you install.
2nd time, cooling down, after you break in your camshaft (initial start up).
3rd time, after you start the engine for the second time.

You should check the torque on ALL of the exhaust bolts at least once a week for the first month after that.

Bolt locks (especially for the exhaust manifold/ headers) are a very good idea, but even they need the first three go arounds with the wrench.
This is a good question!
This is one you don't get very often, but it's one of the most commonly screwed up things on a vehicle.

So many cats, so few recipes...
Junk Yard Genius is offline  
Sponsored Links

Quick Reply

Register Now

In order to be able to post messages on the Off-Road Forums & Discussion Groups forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

For the best viewing experience please update your browser to Google Chrome