Engine runs bad after 30 minutes of driving

[chengny]

I know you have looked closely at your plug wires:

I have an expensive set of 8mm plug wires on and I've checked them over. Also have watched them at night for arcing. Haven't seen anything.

But this was done at idle - correct? It is safe to assume that you have not checked them in real time (i.e. under the actual conditions that exist when this issue pops up).

If you have another (known good) set of wires laying around, do yourself a favor and change them out. It won't take too long, right - maybe half an hour?

Test drive. If there is no improvement, you can definitely eliminate a bad plug wire as the cause.

 

[PrairieDrifter]

Are your fuel lines close to the exhaust at all? If it usually happens when it's warmed up it could be your fuel lines being to close to the exhaust and its heating up the gas and you aren't getting enough gas flow because the gas is vaporizing or boiling somewhat.

 

[Jacknife]

Well he is sure enough right about GM direction. I had to check, I have this same dist. in two trucks. I thought it sounded wrong to leave it unplugged on the 383HT, but for that engine that is what they say. I copied to paste here:
4. Start the engine and adjust the initial timing. If using the HEI distributor P/N 93440806, set the ignition
timing to 10º before top dead center (BTDC) at 650 rpm with the vacuum advance line to the
distributor disconnected and plugged. This setting will produce 32º of total advance at wide-open throttle
(WOT) when using the HEI distributor P/N 93440806. The HEI vacuum advance canister should remain
disconnected. This engine is designed to operate using only the internal centrifugal advance to
achieve the correct timing curve. Rotate the distributor counterclockwise to advance the timing. Rotate
the distributor clockwise to retard the timing.

 

[chengny]

Are your fuel lines close to the exhaust at all? If it usually happens when it's warmed up it could be your fuel lines being to close to the exhaust and its heating up the gas and you aren't getting enough gas flow because the gas is vaporizing or boiling somewhat.

Along this line of thinking - but not heat related/vapor lock due to location/season (February in Montana). Have you looked at whether it might be a fuel flow problem?

Is the capacity of the truck's stock fuel supply system capable of supplying the increased fuel flow - that a fully loaded 383 requires?

Was the fuel pump upgraded when the engine was changed?

If so - and the pump is properly sized for a 383, that places additional demands on the associated piping. This is especially true of the section of piping that connects the pump suction to the fuel tank.

Was the size of the fuel line changed to accommodate the increased fuel flow?

Is it possible that, if any rubber hose is used in the pump inlet piping, it is collapsing under the suction produced by the over-capacity pump?

Are there any old fuel filters in that section of line?

 

[Don5]

I am thinking you got a load of fuel that was low in the tank at the gas station. I suggest you try changing out your fuel filter. Do you have a fuel pressure gauge hooked up inline? I bet you got a load of bad gas and you have some crap in the gas tank that plugs up the strainer.

I could be wrong. but I bet it is something simple.

 

[77 K20]

Since the truck first started running bad I have switched tanks, switched gas stations, used fuel additive and have went thru at least 5 tanks of gas. Doubt it is fuel related at this point.

GM recomended a high volume mechanical fuel pump. (Part number 12355612 is recommended for street performance.) This was installed. Fuel line size was kept the same. I don't make a habit of flooring the truck. So far the issues are at part throttle or just highway cruising. There shouldn't be a huge fuel requirement then. My previous engine was a 400 with a pretty good cam in it. Never had any fuel issues with it.

I woke up early and replaced the spark plugs. (old plugs looked great by the way. Look almost new. Just some light brown on them) Thought I fixed it- as I went out on the highway and drove for around an hour. Truck did great. Then I stopped and turned around. Truck started sputtering right off of idle. It cleared up in about 20 seconds and was fine. I was able to repeat this anytime I wanted... when doing 65 mph or so (3000 RPM) all I had to do was let off completely on the gas pedal for 5 seconds or so, then re-apply throttle. It would stumble. Either giving it more gas, or going into neutral and rev the engine would clear it out and it would be find again.

So I came back home and added a return fuel line. My stock fuel pump had a fuel return line. The new pump didn't. Was thinking that perhaps after cruising at 3000 RPM and backing off the throttle was flooding the carb? So I cut the restricted fuel line fitting off of the stock pump and added a T fitting in line. Went out for a drive. Didn't do anything... though maybe it would allow fuel pressure to bleed off when no throttle was requested.

Now waiting on the engine to cool so I can install a new set of 8 mm spark plug wires. After this the WHOLE ignition system has been replaced- and the "old" system was less than a year old and only had a few thousand miles on it. Maybe something is wrong with the engine...

So not sure what I learned today. It does run better when just running down the highway... better than before. Guess I'm having 2 issues? Ignition related and maybe something about the carb...?

 

[PrairieDrifter]

It seems like it could be dumping too much gas or not burning enough quick enough. I like to gap my plugs at 45 or a little under for a little bit more of a hotter spark

 

[77 K20]

It seems like it could be dumping too much gas or not burning enough quick enough. I like to gap my plugs at 45 or a little under for a little bit more of a hotter spark

These were gaped at 40 as per the engine spec sheet.

If the carb was dumping too much fuel down all the time wouldn't my plugs have looked black and kinda wet?

 

[Skweegle89]

Are you running a carb spacer or just the thick gasket?


Sent from an old rotary telephone.

 

[77 K20]

Are you running a carb spacer or just the thick gasket?


Sent from an old rotary telephone.

Just the thick gasket. Can't remember off the top of my head, but I found quite a few articles about people putting this engine on the dyno, then modifying them after they got a baseline. One place said by running a spacer they lost 30 HP. Someone else also lost around the same amount by running an Edelbrock carb.

GM recommends a Holley 750 or a quadrajet. The Holley they recommended was close to $700 if I remember correctly.

 

[Skweegle89]

Just the thick gasket. Can't remember off the top of my head, but I found quite a few articles about people putting this engine on the dyno, then modifying them after they got a baseline. One place said by running a spacer they lost 30 HP. Someone else also lost around the same amount by running an Edelbrock carb.



GM recommends a Holley 750 or a quadrajet. The Holley they recommended was close to $700 if I remember correctly.

I have heard mixed reviews about the spacer. I had one on my old motor. I was just curious about heat soak. It doesn't seem very likely, but the fact that it only happens after extended hwy driving baffles me. I wouldn't think it's ignition related. My money is on a fuel/fuel delivery problem. I definitely don't think the solution is a $700 carb lol. This thread has my attention/curiosity though. Hope you get it figured out soon.


Sent from an old rotary telephone.

 

[350runner]

are there any plug wires touching the engine?

Sent from the dust in front of you!

 

[Jacknife]

I think something is malfunctioning (possible Carb). Everyone needs to keep in mind that it ran fine for 6000 miles. So it is a matter of finding what is no longer working the way it should. Appartently the install & part selection is okay if it ran that long without any problems. I would have some that really knows what there doing with a quad. jet look at your carb. Or contact Jet for there opinion.

 

[PrairieDrifter]

If the carb was dumping too much fuel down all the time wouldn't my plugs have looked black and kinda wet?

Not necessarily if it runs good after it starts to run ****** it will most likely burn all of the fuel, maybe why your plugs were slightly brown? If you can somehow get it to run ****** and then turn it off you could pull a couple of plugs really quick and see if they're wet

 

[77 K20]

Temperature was 20 degrees today- so heat soak is kinda hard to do.

None of the spark plug wires are touching anything metal, nor the engine.

I'm going to put the new plug wires on tomorrow anyway just to rule that out. The new ones seem nicer than the older ones. I'll keep the old ones as spares.

I tried to get a hold of JET in December. Their customer service sucks. Tried every day for a week and never got a hold of anyone. They don't respond to emails either. In Jan/Feb I've been traveling out of country for work so gave up on contacting them. I'll try again, and try to find someone that knows quadrajets locally.