Engine runs bad after 30 minutes of driving

[77 K20]

This thing is really starting to piss me off. Pretty much everything under the hood is new. After putting in the new engine it ran great for 6,000 miles or so. Then a while back I went on a 3 hour drive. Did great for 2.5 hours until I had to stop and make a right onto another highway. It started mis-firing. When I shut it off and let it cool down it will run fine again, but after about 30 minutes it will start missfiring/shuddering/shaking.

All ignition parts are new (6,000 miles on them). I replaced the ignition coil. No difference. Just replaced the cap/rotor/coil. No difference.

WTF? What can cause it to run great, then after 30 minutes of driving runs horrible? And then when you stop it runs fine at idle? When power braking it or putting it under load is when it really breaks up.

4-5 tanks of gas have been run thru it since the original incident also...

 

[bucket]

Ignition module? Cheap plug wires? What do the plugs look like?

 

[77 K20]

Ignition module? Cheap plug wires? What do the plugs look like?

I just replaced the ignition module with one made by Mallory. Made no difference. 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. Plugs just have some light brown deposits on them. They look good... as they are almost new. Plugs are AC Delco recommended by GM for the crate engine.

Also the distributor is a GM performance unit that the engine paperwork recommended. They say do not run a vacuum advance on it either... so I have that disconnected.

 

[rich weyand]

Also the distributor is a GM performance unit that the engine paperwork recommended. They say do not run a vacuum advance on it either... so I have that disconnected.

That's a GM screwup. Hook up the vacuum advance. BTDT. Then set the base timing at least 12*BTDC.

 

[77 K20]

That's a GM screwup. Hook up the vacuum advance. BTDT. Then set the base timing at least 12*BTDC.

The paperwork said 10*... but I'll try 12*. I thought it was real odd just to do mechanical advance.

But what would cause it to run bad after it is fully warmed up? It used to run great...

 

[350runner]

X2 what bucket says. The reason they state not to use the vacuum advance is because the amount of advance it provides needs to be limited. Without doing so will send the engine detonation and void the warranty. Check for loose plug boot connections, try your old coil, the button the coil sits on, deposits on plugs? Those are just a few extra things to check

 

[rich weyand]

No, the instructions for the 350/290 crate actually say the engine was designed for no vacuum advance, which is a load of hogwash. Hook up the vacuum advance to manifold vacuum, not ported vacuum. If it's an Edelbrock carb, that's the driver's side port, not the passenger side port.

OK, so what happens when the engine warms up?

Thermostat should open up. Sure that it's working? Do you have a temp gauge? Is there any indication the engine might be overheating?

The intake manifold will heat up. What carb is on it? The Edelbrocks are sensitive to being overheated. Do you have a heat isolator gasket under the carb?

The choke should open up? Is it opening? All the way? Choke adjustment depends on outside temperature. Have you adjusted the choke since it got cold? They usually need to be adjusted slightly seasonally.

Also, a long trip at temp can cause some things to get loose that were torqued OK when they were cold. Heat cycling can make things get loose. Have you checked that the distributor is still clamped in position?

Edelbrock carbs have pop-up pistons that adjust the mixture from the cruise circuit to the power circuit. These gum up and stick. Every 5000 miles or so, you should pull them and wipe the pistons clean and spritz some WD-40 in there to clean up the bores.

There are several possibilities here. Something changed/moved/clogged up. It may have nothing to do with the long trip, it could have just happened at the same time.

 

[77 K20]

The engine is a GM HT383.

The carb is a new JET quadrajet. It has the heat isolation gasket under it. Electric choke on it is working properly. It was just re-adjusted last month. After driving it today I verified the choke plate was open after driving.

I have an aftermarket VDO coolant temperature gauge. Thermostat is a 180*. It warms up and maintains that temperature. Engine runs fine when it is first at 180*, then probably 15 minutes later all of a sudden you can hear a slight miss in the exhaust. Then over the next 5 minutes it keeps miss-firing more. Any acceleration at that point or going up a hill results in the truck shaking/shuddering. With the throttle held very steady you can feel it cut in and out every few seconds. There is next to no power. It won't "clear out" if you romp on it, or switch to neutral and rev it. Then when I stop it idles just fine. Only under load or driving it starts acting up.

Today when putting the brand new cap/rotor/coil on it I checked to make sure the distributor clamp was still tight. It is. Timing is still set at 10*.

I also went thru and verified the spark plug wires were seated properly at the cap and the plugs.

 

[350runner]

Sounds like one or two plugs are giving you trouble. I've found missing under load tends to be plug related or cam related. I highly doubt it's your cam tho. Just sounds like one of those plugs isn't firing properly.

 

[77 K20]

Sounds like one or two plugs are giving you trouble. I've found missing under load tends to be plug related or cam related. I highly doubt it's your cam tho. Just sounds like one of those plugs isn't firing properly.

I do have another set of AC Delco plugs. I'll try those next. But shouldn't the missing under load happen right after the engine is started and cold also?

I just get the feeling the truck is trying to tell me something, but I'm not getting it.

 

[350runner]

Usually with plugs the hotter the engine gets the worse the miss. Did you re-torque the intake bolts?

 

[77 K20]

Usually with plugs the hotter the engine gets the worse the miss. Did you re-torque the intake bolts?

No- didn't re-torque the intake bolts.... is this a common issue for intakes (engine has vortec heads)?

 

[350runner]

I always re torque aluminum intakes after a few heat cycles. Not so much a problem but more so preventive maintenance.

 

[rich weyand]

Geez, they tell you no vac advance on a 383HT?! That's wrong.

You could have some bad plugs. If the ceramic is cracked, it can open up when they're hot, arcing to the head, and close back down when they are cold. They can come cracked new out of the box, or they can crack in service. Which plugs are they?

 

[77 K20]

Geez, they tell you no vac advance on a 383HT?! That's wrong.

You could have some bad plugs. If the ceramic is cracked, it can open up when they're hot, arcing to the head, and close back down when they are cold. They can come cracked new out of the box, or they can crack in service. Which plugs are they?

Ok... the cracked ceramic sounds like it could be the issue. I hope that's it. I'll change them tomorrow. Does it discolor the ceramic or anything when this happens? I'll inspect the old plugs so see if I can learn anything from them.

GM calls for AC DELCO R44LTS gapped @ .040"