Hi all,
I'm having an issue with my engine where, at WOT, it starts to buck/hesitate, etc. There's power loss, and a "helicopter" sound to the engine.
First things first, it's not from being lean. With my Sniper EFI system, I can data log an entire drive and look over it on the computer later (as well as just looking at the readout on the handheld while driving).
Whenever this issue occurs, the AFR is usually right on, if not a little rich.

The issue is purely throttle based, and has nothing to do with RPM, and increases the harder I press the pedal. It usually doesn't occur until the vacuum drops below 2 - 4hg.

I was wondering if it was something to do with my ignition, but I would think that would be RPM based, not throttle based.
Throttle would mean an increase in engine pressure, so something to do with the valves? That got me worried that it's an issue with a cam lobe or something (last thing I need right now), but after looking up all the symptoms online, nothing matches.

I'm really not sure what to look into to diagnose this as I've never experienced it before, and search results online are coming up empty.
Any ideas?

Thanks

 

Your last reports had the engine running very nicely.
Is this something that suddenly started?
What ignition system are you running?

If the spark is weak, as the cylinder pressure increases with throttle opening the spark can blow out.

 

Can you verify all your sensors are working correctly? Make sure there isn't an issue with the TPS, ECT or MAP sensor. I've seen a lot of WOT issue's that were TPS related.

PMuller if it was the spark blowing out could he tell that by his data logs. I would think if that was the case the AFR would go rich and the computer would cut fuel to try and lean it out.

 

fordman
He should be able to read his tps percentage to see if it still correlates with the throttle position.
I agree that if there is misfiring the O2 readings should suddenly change but it certainly isn't firing correctly to create bucking and power loss and the O2 reading doesn't seem to change.

I was more interested if this was some that happen all of a sudden or gradually.

 

Thanks for the replies everyone.

The engine does run very nicely, and is extremely smooth. It's not something that just started. For the first few weeks of driving, I never really got into WOT, as I wanted to ease into it. Then, the more I gotten on it, the more I've noticed the issue.
It's something that I figured would "tune" itself out from the self-learning, but just isn't. Then the more I look over all the readouts/gauges, the more everything looks normal and healthy. However, the way the engine feels at WOT is anything but. So, I'm scratching my head.

I have the DUI HEI style performance ignition. When the engine/ignition was in my other Bronco, and it had a standard 4bbl carb on it, I got into WOT plenty of times, and never noticed an issue. So, I don't *think* it's the ignition (especially since it's throttle percentage based, not RPM based), but that doesn't mean it isn't.

I hadn't thought about a weak spark. That would actually make sense on a number of other things. My gas mileage is not good (about 10 in town and 12 on the interstate vs my previous 13 / 17) so a weak spark very well could be the culprit.


As far as I can tell, all the sensors are working properly. I have the TPS, ECT, and MAP displayed on my handheld and can watch them continually, and they're all functioning very well.

As for the AFR, that is the one symptom I have noticed when it does this. It goes rich, like 9 - 10AFR. I kept thinking the system would tune it out, but it hasn't. Maybe it is spark blowout??

It's not TPS based, but MAP based. I've had it around 30 - 40% TPS and had the issue as well as 60 - 70% (depending on my gear / speed) but it's always when the MAP approaches atmosphere (around 90 - 94)

Yes, the AFR erratically goes rich.

 

Check the supply voltage at the distributor while the engine is running and see if it is 14 volts.
The ballast resistor wire coming from the ignition switch may still be there dropping the supply voltage.

 

This is how I have my ignition set up. I use the ignition switch wire as the "signal" wire for a relay. Then, the relay sends power straight from the battery to the ignition, so that it always has full voltage through a good, heavy 12ga wire.
My EFI unit is tied in the same way and I can see a readout of the battery voltage. Looking over logs, it's still at around 14.0 - 14.4v during this scenario.



Could it be something like a coil / ignition module issue?

 

Great job on the power circuit.
I would get a spark plug and connect it to a plug wire, lay it on the engine block and see what the spark looks like with the engine running.
Leave all six plugs in the head so you don't blow fuel out to the test plug.

 

I'll look into that and report back.
I might also just take the ignition module down to the parts store and have them test it 5 or 6 times and see how it does.
I'd much rather this be an ignition issue than anything else. I just couldn't come up with how it could be, so I've been a bit worried that something's wrong with the engine itself.
But a weak spark makes sense, in a lot of ways, now that I put the pieces together.

 

Something else to check is the condition carbon button on the distributor cap. It's the part in the distributor cap that transfers the spark from the coil to the rotor. On the few gm's I've owned in the past they all had issues with that. It's probably not the issue. But it's easy to check and cheap to replace. So you might as well take a look at it. It would be nice if it's something that simple.

 

So, last night, I pulled off the cap and ran tests on all of the components. For starters, the button on the cap, as well as the posts looked just fine. A little bit of visible usage, but nothing major that I couldn't scrape off with my fingernail.

The primary resistance on the coil needs to be between 0.6 and 1.5 ohms. Mine tested at 1.3.
The secondary resistance needs to be between 6,000 and 10,000 ohms. Mine tested at 8,400.

So, the coil checked out within spec.

However, the magnetic pickup coil did not.
It needs to be between 800 and 910 ohms. Mine continually tested at 1100, which is definitely out of spec.

After doing some reading, I found this:

That seems a lot like what I'm dealing with.


I haven't had a chance to test the spark and look at it, since I don't have a spare spark plug, but either way, it looks like I need to replace the magnetic pickup coil. Hopefully that's all it is.
It's getting worse, too. This morning, the symptom was arising under lower load conditions.

 

Performance Distributors said that 1100 Ohms isn't that far out of spec and shouldn't cause an issue. Several sources online I saw said 500 - 1500, as well.
They did offer to run it through their machine and check it over. However, that requires shipping it to them, and about a week or so of downtime.

I'm now still thinking it's ignition related (which I'd much much much rather deal with than any internal engine issue) since sometimes the engine will just "wake up" and will feel healthy and perky. On the flipside, I had the bucking/surging this morning happen at only partial throttle, so something's going in and out.

Need to figure it out before I'm left on the side of the road.

 

Have you pulled / checked your sparkplugs? Could the higher spark energy from the DUI distributor erode electrodes faster? It's not unheard of for the insulators to be cracked on a "new" plug... not common, but does happen.

Do you still have heat to the underside of your manifold?

 

I had them out a few hundred miles ago and didn't see any issues. I'll be pulling them here again soon just to do a full compression test, and see if anything is visible.

No, I don't, as I don't think it's necessary with the Sniper EFI system. It adjusts/compensates for the AFR by the millisecond, as well as for hot/cold air.
If it was an issue, I don't think it'd cause a bucking/pulsing and drop in power. I've had rich bogs before and they feel very different. Sorta just "soggy" performance.

 

Update:

So, randomly, on the way home from work today, the issue was completely non-existent! I put the vacuum gauge in the cab so I could watch it while driving (I've been watching the MAP sensor readout on my Sniper handheld). I wanted to see if the needle pulsated with the bucking of the engine. However... I floored it and the needle dropped to 0 and the engine just pulled like a banshee, smooth as glass. AFR was a nice, healthy and steady 13:1.

For the first time in weeks, and almost 1000 miles, I could press the pedal completely to the floor while accelerating without the pulsating/bucking in the engine. RPMs didn't matter, I tried it at low RPMs (1000), pulling onto the interstate (4000 - 5000), etc and it was smooth and effortless for the whole half hour drive home. I tried over and over and over again, flooring it, punching it, etc. Nada. It's the best it's ever driven.

What gives? It was acting up on me on the way to work this morning.

I had planned on pulling in this evening and seeing if I could retrofit my old DuraSpark II unit to see if it cured the issue, but can't go about that now that it's not there.

Any ideas what would just happen to magically make this go away?
Obviously, if it can clean itself up, it can come back just as easy. I've learned from too much experience that engine issues don't just sort themselves out.