365 GTB/4 engine runs rough after being parked 1 hour with warm engine

My Daytona normally runs great. If I stop for gasoline and re-start the car in 10 or 15 minutes everything is fine. On the other had if I stop for lunch for an hour or two then the engine will run rough upon restart. This will last for the first 10 to 15 minutes of driving .... and then it will be fine. This is very annoying and a bit dangerous as much of my attention is in keeping the revs high enough to avoid the engine from stalling. I assume it is either vapor lock or fuel percolating into the intake manifold while the car was parked. Anybody else experienced this and how did you solve it?

Ivan

 

I haven't experienced your symptoms at all. But I did ceramic coat my headers to keep the heat down in the engine bay. I wanted the old school look of white, which discolors quickly. Heat is still heat, I just moved it to the exhaust system from the engine bay.


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It would be worth checking your plugs are not fouled as that’s simple. If ok, it would be worth changing you condensers on the distributors, they have bad running symptoms when they start to fail and they are known to fail.

If it was vapour lock, I can't see why it would run well for 15 mins. after a stop, it would be hard to start, then run rough for a while until it cleared or the car cooled down.

 

Do you know how many miles the car/engine has done since new and/or being rebuilt? Does it blow visible smoke out of the exhausts?

 

32K miles since new. No smoke at all. Water temperature between 180 and 190F.
It takes more effort to start the Daytona after being parked for an hour and you can smell the raw fuel. Luckily, I installed a gear reduction starter and the engines spins well therefore it has never failed to start.
I've had cars where the engine bay heat makes the Webers percolate fuel when the car is parked. When that happens you can see the fuel boil and go into the intake manifold. Not sure if this is what happening on this car; something for me to test.
Ivan

 

this then has nothing to do with vapor lock

replace the condensers first. fast to do and not expensive

 

Ivan: Just my two cents worth. Make sure the return fuel line from the fuel log to the tank is open and allowing excess pressure to "bleed off". Today's gas is notorious for boiling under hot-soak conditions in all the older cars. We fight it all the time.

 

Heat soak is what I thought Ivan, but I wanted the real tech experts to talk before me, you can trust DW46's competence blind

 

It has electronic (breakerless) ignition .... no condenser to replace.

Ivan

 

Very good suggestion as this would explain the symptoms! All excited I checked the fuel return system, wanting it be fail and therefore give me something to fix.

First, I disconnected the return line located to the left of the brake booster. Turned on the ignition and fuel came out ... as it should. Then connected a hose to the return pipe, opened the gas cap, and blew on the hose with my mouth. I could hear air coming out of the gas tank when I blew; the return line is not clogged. Reconnected everything, turned on the ignition and I can hear fuel returning to the tank. Darn, that is not the problem

Ivan

 

Have you done a compression and leakdown test? Also check the compression after the hour/interval the car has difficulty starting.

 

Ivan: your testing procedure for the return line is correct. Sorry that did not solve it. Are both pumps working? My car will definitely run a little ragged after a hot start, but it usually is OK in a minute or two. What size Idle jets are you running?

 

A compression/leakdown test was done a few years ago, before I purchased it. It was very good and not many miles have been driven since.

I cannot see a low compression could manifest itself with the problems I am having. How could low compression cause the engine to run rough for 15 minutes and then run just fine for hours?

Ivan

 

Just to eliminate that at the temperature range you are experiencing problems, the piston rings/bores and valves/guides are sealing correctly?

 

Although the fuel return suggestion did not work, I think your focus on fuel delivery is the correct one. I was not aware Daytonas have 2 fuel pumps (I am mainly a Maserati guy) so that is something to check. I also need to look at the fuel delivery volume to see if it looks adequate. What fuel pressure should I be looking at?
No idea the size of the jets ... will need to pull one out.

Ivan

 

Fuel pressure of 2.5-3.5 psi is plenty. You can hold your hand on each pump to make sure it is working. After you know both pumps are OK, you will notice the "sound" is different than just one pump. Idle jets can make a dramatic difference with today's fuels.

 

Ivan: I doubt you have a fuel pressure problem, and remember the actual operating pressure is determined by the size of the restriction in the return line.

 

I noticed the return line is very small (about the size of a metal brake line). Is there not a pressure regulator in the fuel system?
I put the car up on the lift and both pumps are clicking.
Pulled two idle jets, they are 60.

Ivan

 

Early Daytonas had the "glass bowl" filter/regulator, but later cars do not. We need Timo ( who has a lot more Daytona experience) to chime in on the pressure regulator restriction. I always recalled it was an orifice in the return line, maybe at the origin of the line at the front carb. We have tried all the way from 55 to 70 Idles and found 60's to the the best for today's fuels. Next time the car does this, try to get a steady idle (don't care how fast) and recheck all the mixture screws. You may find some open up more to smooth out and stabilize the hot idle. Engines always tell you immediately when they are lean, but just keep working away when they are rich.

 

So you think it could be idling a bit too lean?

 

Yes, possibility too lean. Just went through a similar situation, though far less drastic, on our car and found about four screws on the left bank wanted to be opened up 1/8 to 1/4 turn when stinking hot. Smoothed things right out.

 

Thanks Dyke,
for your vote of confidence (?) or at least word of support.
Yes, I believe fuel pressure is mainly regulated by the small (metering ?) orifice at the forward end of the fuel rail.
I haven't checked to see if the orifice has different sizing between "early-to-mid" (with glass bowl filter/regulator) & "late" (without "glass bowl") systems, but will do so within a week or two when working on fuel delivery systems on both models simultaneously.

 

I should have mentioned that my car was built December 1971, US model. I guess that makes it a late model.

 

IMO, mid (for all production), early (for U.S. market examples).
VIN below 15000 ?

 

VIN 14903