I recently bought a 1984 z28 H.O. with ~34K thats been sitting for quite some time. I did a full tune up and rebuilt the carb. Ive been doing alot of research on tuning the q-jet but im still having a couple of problems. I drive 35 miles one way for school (all highway), today the whole way there the car performed flawless. About halfway home cruising at 65 the car starts to buck or surge violently and nothing i do with the pedal will resolve it. I could barely make it to 40 mph. Even shifting to neutral i could not give it any gas without it bogging real bad. I stopped and shut the car off, nothing seemed out of the ordinary. I restarted the car and it went ok for about a minute, then the same thing. This is the second time this has happened and im out of ideas. Anyone have a clue?? I need to fix this. Thanks in advance.

 

Sure sounds like a fuel delivery issue, but you need to verify that. The HO should have an in-tank electric pump - verify that it does first. You can do that by checking the number of wires going to the fuel tank. If it has the pump and you can't hear it running, like when you first turn on the ignition, that's a pretty good indication of the problem. You can also hot-wire the power to the pump with the fuel line off and routed to an appropriate container to check for flow.

These pumps are prone to failure, and having sat for a long time will only make that worse (believe it or not). Your mechanical pump may also be weak. But, if the in-tank isn't working, the mechanical will not be able to do its job properly.

Can we assume when you rebuilt the carb that you put in a new fuel filter? What all did you do with this "full tune up"?

 

So it has a mechanical fuel pump AND an electric in tank pump?? I did change the fuel filter. By tune up i mean all new fluids/filters brakes, plugs/wires, carb rebuild etc. I changed the ignition module in the distributor also. (I was told that could cause it). Im confused as to why I can drive around fine and then all the sudden later on that happens. And once it happens it wont go away until the car sits for some time.

Where would the wires be to see if there is an in tank pump? All I can see is 3 metal tubes coming out of the tank at the top on the drivers side.

 

Also, I ran the numbers on the carb and according to that it is from an 85 non ho. Supposedly there is an H.O. specific carburetor? Could this cause a problem?

 

Nah, doesn't need to be an HO specific carb for it to run. Might want to change up to B hangers and DR rods once you figure out what the real problem is. It'll then perform about as well as the original HO carb.

I didn't have an electric fuel pump at all in mine, but I know they were added to some in order to help prevent fuel vapor issues. It's hit and miss as to whether yours has it or not. Doing what 5-7 says will tell you if you have one.

I vote ignition module - but I never had one go out on me when I had CCC. But I've had one go on me on another car, and after the engine was fully warmed, it ran real rough and misfired. Let it sit and cool down, restart and it would run great till it got warm again...

 

I did replace the ignition module. What about the pickup coil? I've heard people say that it could cause a surging problem. It seems to me like the car isn't getting fuel when it happens. It's like I'm flooring it and letting off then flooring it and letting off over and over. Even if I ease back on the throttle it won't stop. I just replaced the mechanical fuel pump. Still happens. Granted it's hard to check because it won't always happen. I don't thnk I have an in tank pump. If I don't, do I need to install one to solve this?

 

Sounds like a fuel delivery problem to me..
A few posibilities to check:
-bad fuel pump
-fuel lines on tank sender loose and letting air suck in
-dirty intake "sock"inside tank-could remove the feed
line off the engine mounted pump and blow back
through the line with air-if there is an in-tank electric
boost pump,it has no check valve and will not be
damaged by blow back.
-If your car has the electric boost pump,the foam
rubber coupler hose between it and the metal line
may have turned to goo and fell off-letting air suck in
when tank is less than1/4 full or so.

 

Intermittent problems can be the hardest to diagnose. At this point, you can't rule anything out whether it be a fuel issue, or spark. One thing I've learned though, is just because a part has been replaced don't rule it out. I've been sold plenty of defective new parts.

You mentioned that you replaced the ignition module because it was old and it might go bad since it was sitting for a long time. Do you still have the old one? Swap it back in and see how it runs. Your spark plug condition might tell you something. You also said you rebuilt the carb - you would think if something was wrong with that, it would run bad all the time...did you check the fuel pump for proper flow?

If you can get your hands on known good parts that you can change out temporarily, it also helps to diagnose these types of problems. Either way, good luck with it and I hope you get it figured out soon! It'll be worth it once you get it running right. The L69 engines are pretty darn dependable once you get them dialed in!

 

Check for the in-tank electric pump. Crapping out when hot and working when cooled off is a typical "failing" mode for them. If the in-tank is there and isn't operating, it makes it very hard for the mechanical pump to suck fuel out of the tank. A clogged pick-up sock is a possibility, but doesn't fit the symptoms as well.

If you don't have an in-tank electric pump, then the symptoms point to something electrical. The ignition coil and pick-up coil are also known fail-when-hot/work-when-cool items.

 

An easier check for the in-tank pump is to look for the oil pressure switch tee'd off the OPSU behind the distributor. The oil switch will have a two-wire connector on it.

If the eighth digit of your VIN is "G" you 'should' have the in-tank pump.

Pickup coil failure is sometimes accompanied by a bouncing tachometer. The pickup coil is only a few $ but can be mixed up with a worn out dist shaft and bushings. Check your shaft play. A reman dist runs around $100.

 

All my fuel lines look solid. I cant find any sign of an in tank pump. Does anyone have a picture of what I should be looking for?

I replaced the pickup coil today but still no luck. I did notice today though that the mixture control solenoid (blue plug on top?) was clicking while the car was running. Isnt it only supposed to click when the key is on and cars off?

I also heard about something that plugs into the t stat housing from the carb and regulates fuel at certain temps. Anyone know anything about this? I do have a harness plugged into the tstat housing but the wires dont go to the carb.

And one last thing. I noticed while driving, a "clicking" noise that sound like its coming from around the distributor. That area at least. Only happens under load. When I stop, it stops. I dont think its an exhaust leak. Thanks again

 

No pic right now but the OP Switch for the fuel pump is mounted on the brass 'tube' that rises from the china wall right behind the distributor. There should be a small 'can' attached to it with a single wire connection that is for the oil pressure guage. If it has the factory wiring for the fuel pump it will run through a two-wire switch mounted off a 90 degree t-block from the OPSU, right behind the dist.

The MCS is supposed to click when the engine runs. It cycles at 10 Hz (10 times a second). The ECM varies the dwell of the MCS, which is the amount of time during its cycle that the rods are down in the jets. This is how it adjusts the fuel mixture.

There are two bungs on the water neck. One is for the Coolant Temp Sensor (CTS) which tells the ECM the water temp. The other is a Thermal Vacuum Switch for the Early Fuel Evaporation (EFE) system. It closes a valve in the pass side exhaust Y to force exhaust through intake manifold passages and out the driver's side exhaust. It helps warm the engine during cold starts. The Vac lines to this valve are worth checking as they're subject to some harsh conditions and are quick to fail.

Nothing in the T-Stat housing that regulates fuel to the carb. The ECM WILL command a cold start dwell to the MCS to richen the mixture during cold starts but nothing here that would likely cause your problems.

A definitive test for the presence and PROPER FUNCTION of the in-tank pump is to remove the rubber fuel line going to the mechanical pump and place it into a container. Start the motor and verify that it's filling the container. The motor should idle for a few minutes with just the fuel in the carb bowls.

A more thorough test might be to install a fuel pressure guage in the fuel line and tape it to the windshield. See what the fuel pressure does when the problem re-occurs. You'll have to tee it into the rubber line between the fenderwell and the mech pump. This would directly measure the pressure from the in-tank pump. These pumps are just helper pumps though. I've only measured 1-2 psi from them and this pressure NORMALLY varies slightly as the needle opens/closes on the carb.

When you replaced the pickup coil was there any play in the shaft?

 

When i replaced the pickup coil, the distributor seemed fine. I dont think thats the problem. I dropped the tank last night to see what was going on in there. There is NO electric pump. But I did notice that the sock was really dirty, you couldnt even see light through it. Im thinking of removing the sock and adding an additional in line filter.

The only thing I have not replaced is the ignition coil. If removing the clogged sock doesnt fix this ill replace the coil. If that doesnt work im gonna light the damn thing on fire...

Oh yea almost forgot. I found a small (approx 1" diameter) black plastic cap just sitting in the gas tank. I have no idea what it is. It opens up pac man style like its meant to clip around something. Anyone know what this is?

 

did replacing the sock fix it my car is having a similar problem when its in neutral its fine revs up but when i put in drive it boggs down bad and sputters sometimes

 

No, no it didnt. I replaced the ignition coil today and still nothing. Im out of ideas. Im getting a cap and rotor tomorrow even though they seem fine. I dont know what to do anymore....

 

sounds like it might be time to take the carb apart and
clean all the passages and check the float and valve
over real good.. could also check for a bad connection
to the mixture control solenoid or an electrical problem
cutting off power to the MCS.
If you can find one,could try a different carb.
Also recheck the little filter in the carb inlet.
Years ago,i had trouble with fuel flow in my '69 camaro
-took the inlet hose off the fuel pump and there was a
grasshopper stuffed in there(when i got the car the
gas cap was missing)

 

If it's an automatic, it could be a sticking converter lockup solenoid. Just unplug it from the tranny and see if the problem goes away.

 

Well, it didn't surprise me that you didn't have an electric fuel pump. Most people assume that since it's a code "G" car (8th vin digit), it'll have an electric pump. '83's are even worse. A kid at the local Autozone here told me there wasn't any such thing as a code "8" car - but that's what mine is - the "8" code was only used on the 83's ...they didn't use the "G" code till 84.

Even funnier, their computer says the code G car is a 305 TBI car....funny how they list a carb as one of the replacement parts under the G code though...

DoubleV might have something there...I had my old Z24 Cavilier do the same thing when it's TCC solinoid went out. It would chug and bck like crazy when slowing down coming off the highway exit ramp because the tranny wouldn't downshift on it's own. But I'm not familiar with the automatics on these cars...

 

Hmm I never even thought about that. Where is that located? Do I have to drop the pan or is it outside?

 

Still sounds electrical. There's not much in the carb that would cause these issues AND be intermittent. The motor will run with either, or both, the MCS and TPS unplugged (albeit rich).

One simple thing to check would be the pressure relief valve on the tank. If it fails the tank can create a vacuum and starve the pump for fuel. Problem can seem to come and go. To rule it out simply loosen the fuel tank cap. Air rushing out is NOT a sign of a failed relief valve though.

Cap, rotor, plugs and wires are routinely serviced tune up items. If you don't know when they've last been replaced, or it's been a while...

Rule out fuel. If it does it again pour fuel into the carb to fill the bowl (you can add it into one of the vents near the IAB). If it immediately clears up chase down the problem between your pump and tank. If no change look elsewhere.

 

Lock up can't engage in first or neutral. You stated above it wouldn't rev in neutral.

 

That's when I was already at highway speed and the problem occured. I was doing ~65 when it started and then I put it in neutral and couldn't get it to rev. Could the converter stay locked up when I take it out of gear?

 

Highly unlikely and even if it somehow stayed locked, when in neutral there would be virtually no load on the engine from the drive train.

 

had the same problem..but mine was manual transmission..it was the distributer cap try that

 

Ok I found ALOT of crap in the tank. Cleaned the tank out real good and replaced the cap and rotor. Sofar so good. Havnt had a problem yet but I just started driving it yesterday. I'm having some problems setting my idle though. It seems like it's either way to high (almost 1500 in gear) or it's too low and choppy(about 400-500 in gear). I can't seem to find that happy medium. While I'm driving it feels like I'm using cruise control all the time. Could this be the mixture screws?

 

This is a ccc-qjet? The mixture screws should have been set to where they were originally and the mixture fine tuned with a dwell meter and the IAB. When you re-built it did you re-adjust it for dwell at idle? TPS is near 0.40 volts at idle?

Idle varying that much can be a vacuum leak. Intermittent high idle can also be caused by sticking choke linkage or the dreaded worn throttle bushings. I've also seen the throttle bump solenoid stay engaged with high TPS voltage at idle.

 

Yes it is a ccc qjet. I don't have a working dwell meter currently. When I got the car, all the carb adjustments had been messed with and I'm pretty positive they were wrong( one mixture screww was a turn out and the other was almost 4) I have them both 3 turns out now. I set the float to the specs in my rebuild kit. And I have the idle air bleed at 4 1/2 turns out from the bottom. I didn't have the guage to set that, but the sheet in the rebuild kit suggests that to start. Choke linkage doesn't stick and seems to work properly. High idle when choke is on is at about 2k( sticker under the hood says 2200 so it's close). I will check the TPS voltage whn I get home.

 

Your carb may be out of adjustment but it's unlikely your primary problem. I'd check for vacuum leaks first, maybe cap every line on the carb then reconnect one at a time see if the idle changes. Also verify that the throttle is resting on the curb idle set screw when it high idles.