Mystery hesitation under load, worsens over time

[rundwark]

Hey all, I have a real – to me – mystery on my hands.

 

'73 240Z w/ '71 L24 with roundtops, Mallory dizzy, MSD 6A, electric pump outside fuel tank, mechanical aftermarket cam-driven pump.

 

Started developing a hesitation under load. First only happened near redline in 1st and 2nd. Now manifests itself during highway driving. Gets worse over time during a drive. First you notice while overtaking in third, then even maintaining speed in 4th becomes a challenge. After idling for a while, it's better for a bit. Idle is also unstable in general since these issues started happening, as if it misses on one of the cylinders every now and then. Idle is especially shitty when I just come off the highway, it will fluctuate heavily around 500 rpm and almost die without gas, while before the highway drive it will have idled around 1k but still fluctuate a little.

 

Water temp is good. Timing is the same as before the issues happened. I tried changing the timing, and changing the jet settings at each carb to richer and poorer. I disassembled, cleaned and reassembled the SU's, centered the jets, cleaned the air filter, replaced the spark plugs and ignition coil. No change with any of these.

 

When the engine gets in this state it also sounds different at all RPMs and loads. It sounds coarse, as if it needs to clear its throat. I thought I was fouling the plugs, but immediately after changing the plugs, revving the engine in the auto parts store parking lot it still hesitated. Old plugs looked alright too. Two were a little pinkish but no black soot on any of them.

 

My theories:

 

1. A clog in the fuel system. The issue first manifested itself when I ran the fuel tank lower than usual. For a while after that, it seemed like the issue was worse on a half-empty tank than on a mostly full tank. I don't really see how this theory holds up with the fact that it does get worse over time during high-load driving, but it's all I can think of. Also, I can see the fuel level in the in-bay fuel filter change when I turn the electric fuel pump on and off (it's on a switch), but see no change in how the engine runs.

 

2. Mechanical fuel pump marginal. It feels like I'm running out of fuel at the high end. But I'm not sure how this would get worse after driving for longer, so that doesn't compute for me either.

 

3. Some internal engine issue? Like a bearing that's marginal? I track the car and don't have a baffled oil pan or anything fancy like that. So oil starvation during a track session is possible. But with the problem gradually getting worse over 500 or so miles, I'm not sure that makes sense either. I would expect it to hesitate and then seize, not cool down and run decently for a while again.

 

4. MSD 6A box marginal. I don't see how to test this really.. I did leave my replacement spark plugs at stock plug gaps instead of the bigger gaps and saw zero improvement. So I don't think this is it.

 

5. Valve train issue? I don't know how this could cause these symptoms but adjusting valves on these cars is like zapping the P-RAM on an old Mac, the first thing anyone does while troubleshooting.

 

I have a freshly built engine sitting in my workshop, waiting for me to drop it in, but really want to isolate and understand the cause of the problem before I potentially expose it to the same issue.. this can't be good for the engine.

 

Anyone here have ideas on how to further diagnose the issue? I'm thinking I'll do an oil change tomorrow, to see if there's any shards in there..

Edited March 11, 2017 by rundwark

[turbogrill]

If this would be EFI then sounds like fuel, not sure about carbs.

[rundwark]

Oh one more additional note: I'm pretty sure there's no vacuum leaks to explain the uneven idle part of the issues. Disconnected brake booster: no difference. Sprayed carb cleaner around intake manifold, around throttle shafts: didn't do anything.

[jasper]

Check for fuel pressure loss.  Also try driving with fuel cap loose, may be improperly vented tank.

[rundwark]

Thanks turbogrill and jasper. I'll focus on fuel-related stuff in my troubleshooting, will try driving with the fuel cap loose tomorrow

[Enzo250gto]

Hi Mate, Check out the rubber diaphragm in the mechanical pump. If it has a small split it will still supply fuel at low rpms but get worse as you drive. As the split get bigger fuel starvation will get worse. Also check you oil and see if it smells of fuel.

Regards

David.

[rundwark]

Thanks David! I've been suspicious of the mech. pump. It's one of those generic aftermarket ones. I have a replacement one on the shelf, will install that tomorrow as well, after trying to drive with a loose gas filler cap.

[grillhands]

I had an issues with my msd on my old turbo civic. Turns out that having it mounted on the firewall without a cushion to absorb vibration rattled some components in there. I went back to stock distributer and the problem stopped occurring. Could that be the culprit? I don't know but worth a try.

[turbogrill]

Ever cleaned the tank?

 

My tank was TERRIBLE, seems like this is common. More rust than you can think of. Big pieces of rust in the pump

[Chickenman]

Have you checked the spark quality yet? MSD 6 boxes are known to fail in lots of weird and wonderful ways. They don'y always fail completely. 

 

Pull the HT lead out of the dizzy and hold it a good 1/2" away from a ground point. Have a helper crank the engine while observing spark. A healthy MSD Ignition will easily jump a 1/2" gap and should give a sharp white/blue spark with a distinctive " Snap " sound. Any weak or yellowish spark is a sign of failure. Check it when the engine is cold and also when it's hot. Electronics often fail when thye get hot, but may work fine when they are cold. 

 

A weak spark will manifest itself more under higher  loads. They have enough juice to fire engine at idle, but fail to properly ignite fuel mixture under load 

[rundwark]

Ran it with filler cap loose: no change.

Blew out rear fuel filter: it wasn't clogged, no change.

Replaced mechanical fuel pump: no change.

Removed MSD ignition box: no change.

 

Next up: either installing a wideband, which I've been putting off or checking floats on the SU's, to see if they're sticking or something like that.

[BJSZED]

Didn't see that you checked the coil ? Try exchanging it for a known good one. Also take a look at your plugs if you haven't.

When it loses power is it sudden and jerky like ignition or smoother loss of power like fuel ?

 

Sorry, just re-read that you changed coil and plugs..oops

Edited March 12, 2017 by BJSZED

[rundwark]

The under-hood fuel filter is only one quarter to half full most of the time with the engine running, I can't remember if it was always that way. Is that normal?

[rundwark]

Hey BJSZED, its set-on is fairly gradual, I can usually tell that power tapers off a bit at first rather than hitting an on/off switch directly but if I stay on the throttle or move past the point where power tapers off, the problem can get pretty aggressive, bucking the entire car as it rapidly goes between having and not having power.

[Chickenman]

The under-hood fuel filter is only one quarter to half full most of the time with the engine running, I can't remember if it was always that way. Is that normal?

No, that doesn't sound normal. 

[Chickenman]

Hey BJSZED, its set-on is fairly gradual, I can usually tell that power tapers off a bit at first rather than hitting an on/off switch directly but if I stay on the throttle or move past the point where power tapers off, the problem can get pretty aggressive, bucking the entire car as it rapidly goes between having and not having power.

 Ah... now that tells us a lot. Definitely sounds more like fuel supply issue now. That plus the half full fuel filter is pointing towards fuel. You mentioned in your first post that you have both an electrical and a mechanical fuel pump. What brand and model is the electric pump? Where is it mounted ( Puller or pusher ) 

 

Check for pinched lines or kinked lines.

 

As I understand, you haven't checked the float level on the SU's yet?  That step should have been one of the first things you did. Also check the Needle and Seats for any obstruction.

 

You should be installing a fuel pressure gauge on the car and monitor in real time. You can temporarily mount one outside the car using a long hose and secure it outside the windscreen.  That will eliminate a lot of the fuel variables. Fuel volume issues will be worse under sustained high loads ( 3rd and 4th gears ) up long hills. Car will often rip through 1st and 2nd, then start bucking under longer loads due to lack of fuel volume and pressure. Unfortunately a weak ignition system can cause the same problems.  

 

BTW, you say you removed the MSD and replaced it with something else. But did you check the spark quality first? That is important and you haven't answered that question yet ( that I can see ) . You could have something like a  coil HT lead that has gone bad. Check spark at both coil lead end and spark plug lead ends.  You have to eliminate things one by one by testing operation of systems as a whole. Otherwise you can chase your tail for a long time. 

 

Not to get off track. But what brand and type of plug leads? Just curious.  

Edited March 12, 2017 by Chickenman

[rundwark]

Hey Chickenman, that's very helpful, thanks!

 

Electric pump: not sure what brand or how old it is. It sits right below the fuel tank so I guess that makes it a pusher.

 

One other thing I didn't mention in the original post: one of the POs removed the expansion tank, so the fuel tank and line setup is not stock. It ran well this way for the last 14K miles though, before these issues started.

 

Float levels, needle/seat obstructions: I haven't checked any of these yet. Will add them to tomorrow's list.

 

I did check the spark quality today: I took one of the plug leads off with the engine running and held it close to a strut mount bolt, spark was strong, blue, and jumped a good distance. It did seem a little uneven where not every spark seemed equally strong, but it wasn't clear enough for me to be sure it really was an issue.

 

I'm using NGK leads and NGK BPR6ES plugs. I'll swap in a different coil HT lead tomorrow morning to see if that makes any difference. Based on what you and others wrote and the causes I eliminated today I feel pretty confident it's a fuel issue, but still not entirely sure..

[rundwark]

One crucial bit that maybe isn't clear from my earlier posts: the issue is somewhat intermittent. It's not always equally bad.

 

Sometimes I can push the car to redline in 1st and 2nd, sometimes it'll hesitate badly around 4000rpm in 2nd. And once it gets like this, it stays like this for a while. Even if I put the car in neutral and let it idle for 10-20 seconds while coasting, it'll still run crappy once I put it back in gear and put some load on it. If I pull over, and check the fuel filter (it'll be at 50% or so), it'll still be bad right away when I start driving again. But if I let it sit for longer, say 15 minutes, the issue will get less for a bit. That makes it sound like it's heat-related to me. Maybe marginal fuel flow acts this way with engine bay heat?

[BJSZED]

I think you're on the right track checking fuel quantity in the bowls and fuel pressure.

 

When I start my engine cold the fuel filter is mostly full and you can see air slowly entering the line in the form of an occasional bubble probably from having a less than perfect fuel line hose connection somewhere by the tank. After the engine is fully warm the fuel filter is 3/4 full. Car runs well either way but doesn't sound right to have air in there although I only run an aftermarket mechanical pump. Also, I have read that the aftermarket mechanical pumps use a different type of diaphragm material which expands more when warm which can affect fuel flow. It doesn't appear to be a problem in my case. 

 

Like Chickerman said, it's likely a fuel issue but still could be spark. Does the Mallory distributor use points or an ignition module? An ignition module can break down intermittently especially with heat.

[madkaw]

What type fuel filter?