RPM Hanging
 

Holley Sniper on a 455. Having a problem with the RPM hanging up when I let off the throttle to come to an idle or even at a steady cruise. If I jab the throttle it will return to a normal idle. No mechanical interference hanging up the throttle cable. TPS shows 0% when it is happening. Can't find any vacuum leaks. The car just started doing this and I haven't changed anything in the tune. I have a datalog I can email.

If you're certain of no vacuum leaks, are you running a distributor? Hanging weights seems plausible. I'll look at a datalog if you want. Send me a PM with your email address and I'll reply with mine.

I’m using the Hyperspark distributor. Timing is controlled by the Holley. I’ll send you a pm.

Sounds good. Replied

If it just started doing this then maybe investigate the IAC.....maybe it is sticking? What is the IAC value when the hanging occurs?

Fyi, I never received the datalog, that's why I haven't answered, or commented. Also fyi, the IAC IS a vacuum leak, just controlled and a good thing to check.

I'd tend to agree. If this is a brand new symptom, the IAC may need cleaning. That's probably the first thing I'd check. Next would be a vacuum leak that has developed.

Unless you were in your tuning software poking around, I doubt a rapid change in loop rate would be there. In general I'd want to try and find the cause of the issue, but you can use the loop rate down function to try and counteract what is going on.

Sent you the datalog file this morning. Here's a screenshot. RPM is around 1400 at this point. IAC and TPS are at 0%. You can see where it hit the throttle and then RPM comes back under 1000 and IAC is back between 5-10%.

I will have a look tonight after work. I'd recommend having another look for extraneous air sources. The cable being ok is good, but the butterflies could have slop and not be returning to fully closed. From just the screenshot, that's what it looks like. Especially if 10-15 is your normal consistent number on your IAC. It goes to zero at points, without the rpm dropping lower. Its likely getting air from somewhere else intermittently. That's why I'd guess something to do with the throttle plates, or their position. A vacuum leak of the traditional sort would be there all the time. But I'll have a look. Maybe there's a pulsewidth spike on an injector or something of that sort.

In my opinion this is one or a couple things going on.

If you follow the IAC and TPS trace across RPM, you see that RPM refuses to fall against a completely closed TPS and IAC. That means air is getting into the engine somehow. That could be:

1. Out of adjustment butterflies
2. Vacuum leak
3. Leak in the IAC valve

You should almost never see a completely closed IAC, 100% closure would mean the system is trying to kill RPM. At idle you'll have it open slightly. You see this accurately on your log between 35 and 55 seconds.

If this was an issue I was facing, the first thing I'd do is get some carb and choke cleaner and clean the throttle bores, IAC port, basically everything on the unit really well. I would do an extremely thorough check at this point for vacuum leaks at the carb flange and the intake manifold/head ports. If that does not cure the issue, I'd then go about resetting the butterflies by going through the IAC adjustment process outlined in the Sniper manual. Finally, I'd look at replacing the IAC valve/motor as it's likely not acting in accordance with what is being commanded.

I'll pull the unit off and check everything this weekend. I'll double check any other vacuum leaks also. Does the log look like it has excessive noise in your opinion?

I wouldn't say so, it looks like most logs that I've seen if there isn't any smoothing applied.

I'll have to see the actual log to know. I assume the Holley has the ability to adjust the sample frequency (my name for it, don't know the technical term). Mine does, and depending on that setting you can have a pretty spiky "looking" log, but the engine is actually running very well. You can slow it down to where it doesn't have the spiky appearance. Mine samples in milliseconds, so there's a LOT going on in that brief amount of time. If the engine is running smoothly, it's likely a function of the sample rate.

It seems like the rpm hangs when the throttle change is gradual, but when there is a rapid increase in the throttle position it does not hang and returns to normal.

I kind of think that the throttle blades might be hanging when the throttle is closed softly, but when the blades are allowed to snap back notice they close all the way?

Also, what is your IAC ramp down settings? You can lower the Ramp Decay Time to allow for a more rapid return to idle rpm.

I changed the ramp decay last weekend and it didn't make much difference. I'm starting to think that the issue is possibly related to a phenolic spacer that I installed. Maybe something shifted a little or there is a gasket catching the throttle blades.

That's a decent point about the throttle hanging, but assuming the TPS is accurate, the log does not show that as being the case.

I had a similar issue occur with my FiTech system when I had a throttle cable that was fraying in it's sheathing. The butterflies would not return to completely closed. My TPS bore that out however, typically showing about 3-5% open when the throttle would hang.

In this case I don't believe messing with the loop down rate would actually make a difference because as you can see in the first part of the log, the IAC is completely closed. At least it is commanded to be closed anyhow.

I'd be checking the screws that attach the butterflies, as well as the shaft to bore clearance of both shafts. The TPS only monitors the shaft position, not whether the butterflies are actually closing/sealing.
ETA: Depending on the type of phenolic spacer, that can be an issue for sure. I've got a really crappy one(Jegs brand) that I have to retighten regularly. I actually think it's deforming over time, and this reminds me that I need to just replace it.

After looking at the log, I'm still basically of the same opinion. But I'd like to see whatever Holley calls the tune file, where it shows how everything is set, especially the timing map. And if you're supposed to be, or are running closed loop idle, or if that's available.

I will email you the tune file. Here's a screenshot of the timing map.

Closed loop may be another issues and something else I had questions about. I attached the same datalog with CL status turned on. It only goes into closed loop status for a few seconds in a 60 second timeframe. Closed loop is enabled. Min temp is set at 160.

Yeah I saw that, and am trying to determine if that's closed loop idle, or closed loop fuel correction. The 2% does correspond to the .2 difference in AFR at that point. Hoping the tune file will clarify things. If you can use cl idle, it will lock things down, if everything else is right. Still thinking there's an air leak though. And that will cause issues either way.