930 Hesitation/Stutter

Overpaid Slacker · 06-17-2010, 12:56 PM

Gentlemen, I need your help.

The situation is a 1979 930, twin plugs, Electromotive ignition, TiAL wastegate and Kokeln intercooler/exhaust.

Car starts immediately and runs strong; however, as we get to the 4000 rpm area, it feels like it loses a bit of power, and then... stutters or judders a bit. If you put your foot into it, it will come right out of this bouncy/jerky zone, but you can't cruise at 4000 rpm area b/c it's uncomfortable. Once you're above about 4500, it goes away, so it's not like an increasingly bad judder cause by bad wires or what have you.

This is not turbo lag. I start to get boost at about 2800 rpm, and I know what lag feels like. This is something else. It kind of *feels like* fuel is stopping and starting (or being cut out and in) a couple of times a second. I have a HALmeter on the car, and it doesn't show a change in lambda when this happens (but it might not be "fast" enough to detect these pulses).

What I've done (my mechanic has done, mostly) thus far:

Checked all 12 plugs -- they look good.
Checked both fuel pumps -- both have good pressure and flow
Changed fuel filter
Changed fuel line to distributor (it was worn and weak)
Generally tried to tune the car... it idles solid and, other than the 4000 rpm stutter, runs well.

So... a few thoughts/guesses?
The boost pressure sender is not working. The dash gauge stays pegged at 1 when the car is on -- I have a separate boost gauge/controller that I use. Could the boost pressure sender's failure cause such a problem? Is there some sort of integral fuel cut-out that the pressure sender might be causing?

There is another sender "next to" the boost pressure sender, whose function is a mystery to me. If anybody can enlighten me as to that little doohickey's purpose, and what problems it might cause, I'd appreciate it.

Any other ideas/theories would be greatly appreciated. This is the last "issue"with this car, and I'd enjoy it a whole lot more if I could get rid of this stutter/judder.

As always, thanks in advance.

Best,

JP

spuggy · 06-17-2010, 04:28 PM

Quote:

Originally Posted by Overpaid Slacker

This is not turbo lag. I start to get boost at about 2800 rpm, and I know what lag feels like. This is something else. It kind of *feels like* fuel is stopping and starting (or being cut out and in) a couple of times a second. I have a HALmeter on the car, and it doesn't show a change in lambda when this happens (but it might not be "fast" enough to detect these pulses)

You sure you're not pig rich @ 4,000 when the boost enrichment ramps up? That would be pretty typical for a stock WUR

Quote:

The boost pressure sender is not working. The dash gauge stays pegged at 1 when the car is on -- I have a separate boost gauge/controller that I use. Could the boost pressure sender's failure cause such a problem?

Nope, it's simply disconnected.

Quote:

Is there some sort of integral fuel cut-out that the pressure sender might be causing?

There is another sender "next to" the boost pressure sender, whose function is a mystery to me. If anybody can enlighten me as to that little doohickey's purpose, and what problems it might cause, I'd appreciate it.

That is your overboost sensor, it cuts the fuel pumps at 1.1-1.4 bar.

Overpaid Slacker · 06-18-2010, 05:52 AM

spuggy -- thanks.

The HALmeter continues to show about 14:1 when cruising at 4000 rpm, so I don't think it's over-rich. Even under full load, it only gets to about 11:1, and then only for a second.

Maybe the overboost sensor is cutting the fuel? if it has gone all screwy, that might be the issue, no?

JP

304065 · 06-18-2010, 06:28 AM

PJ,

Get your self an LM-2. Yeah, you need another tool like you need a trepanning, right? Nah, the LM-2 is essential to see what is happening in a dynamic way-- you go out, you make a couple runs, you go home, snap a cool one and read back the log file from the comfort of your PC. (You stop, you push it in. You go, you pull it out. Are you gettin' it?)

spuggy · 06-18-2010, 07:54 AM

Quote:

Originally Posted by Overpaid Slacker

Maybe the overboost sensor is cutting the fuel? if it has gone all screwy, that might be the issue, no?

JP

Anythings possible although most people report "dash in the face" syndrome when this trips which doesn't sound like your problem. Factory procedure describes using a footpump and and a multimeter to see where it triggers. Anywhere between 1.1-1.4 bar (~16-20 PSI) would be spec.

Is your boost controller manual or smart and what does it do to the wastegate at these RPMs? Is your boost gauge takeoff above or below the throttle plate? (below can show vacuum, above can "see" transient boost spikes)

If you're reading the same boost @ 4000 as other engine speeds that the car runs fine at, seems unlikely to be the overboost protection.

Eh, you already flirted with the dark side when you deviated from factory spec. Now it wants you to dress up nice, make a commitment, get to know it better..

Might get more/better ideas over in the Turbo section.

Overpaid Slacker · 08:36 AM

Spuggy -

I agree, it's probably not overboost protection.

The boost controller is "smart" and forces the wastegate to open at predetermined boost (not revs, per se). Not sure about its plumbing, but I'll find out.

What's the remedy for a faulty WUR? OK, maybe it's not faulty, but it's not coping with the mods....

Having spoken to my wrench (who has looked at this thread but is not a Pelican), he thinks the WUR may be the issue. is 11:1 "pig rich"? I thought you'd need some degree of enrichment coming on boost.

JP

spuggy · 06-18-2010, 03:45 PM

Quote:

Originally Posted by Overpaid Slacker

Spuggy -

I agree, it's probably not overboost protection.

The boost controller is "smart" and forces the wastegate to open at predetermined boost (not revs, per se). Not sure about its plumbing, but I'll find out.

Well, some electronic boost controllers can use different boost levels - e.g. more boost at lower RPMs, trailing off as RPM rise - which I believe is common practice on many cars turbo'd from the factory - which was why I asked...

Quote:

What's the remedy for a faulty WUR? OK, maybe it's not faulty, but it's not coping with the mods....

Two words: "Brian Leask". That route would get you a rebuilt, modified/adjustable WUR.

Not only can you adjust the hot/cold control pressures, you can tweak the amount/onset of boost enrichment. Not perfectly, but much more than the stock one, which has none whatsoever.

A common problem on the stock WUR is that the boost enrichment circuit slams into pig-rich as soon as it "sees" a few (2-3?) PSI of boost, and the enrichment drops off at the top end of the RPM range.

Even with adjustment on the enrichment circuit, if you set the AFR's "safe" so you don't melt pistons at the high end, the mid-range is often rich to the point of concern about fuel contamination of the oil - to say nothing of power left "on the table"...

One approach is is the "RPM solenoid" mod - also developed and available from Brian Leask - intended to block the boost signal from triggering the boost enrichment in the WUR until a pre-set (but adjustable) RPM range.

This wasn't necessary on my car (8:1, SC cams, Kokelyn, GHLs, K27/HF, Leask adjustable WUR), so took it off again - but some guys struggle to get acceptable mixtures without it.

Much discussion of these modifications and AFR charts on the Turbo forum.

Quote:

Having spoken to my wrench (who has looked at this thread but is not a Pelican), he thinks the WUR may be the issue. is 11:1 "pig rich"? I thought you'd need some degree of enrichment coming on boost.

JP

There's many schools of thought, you'd be better asking someone who knows what they're doing

10:1 or lower is pig rich. 11:1 is pretty darn rich. Ideal ratio for power/ safety margin on a CIS 930 under load is generally considered to be somewhere in the low to mid 12's, I believe. Still rich. But don't take my word for it.

The guy that tuned my 930 in Europe (Bob Watson) has been working on them a long time - he observed that "back in the day" thinking was very different (and hence - I presume - the design of the enrichment circuit), and mentioned that the factory used to advise that a 930 couldn't be too rich if raw fuel wasn't actually running out of the exhaust. LOL.

Current thinking is quite different. He tweaked my WUR after install to lean it out because it was "bogging down, too rich in the mid-range". It was true that you could hear the exhaust note change before. Certainly ran much nicer afterwards.

(You tell Brian L. about your configuration, and he sets up your adjustable WUR for that - erring on the "safe" side naturally, in case folks just bolt it on and don't check AFR's before hammering it.)