bob944

Hi all - I have not been a frequent Rennlister, and I've been out of the track game for years but starting to jump back in. I know the depth of knowledge here is unmatched, and I greatly appreciate any help from the collective wisdom. I've searched for info, but my scenario seems slightly different than other threads I have researched.

I'm struggling with an issue on a new-to-me '86 944 Turbo track car. The previous owner spent a lot professionally maintaining the car, but he has very limited knowledge under the hood. Various shops have done different mods over the years so I'm putting the puzzle together - and dealing with gremlins along the way.

The car has an upgraded turbo and runs an Autothority MAF kit with chips and a Greddy Profec Type S electronic boost controller. It also has a Lindsey (I think) FPR. I believe other relevant components remain stock (wastegate, plugs, wires, coil, fuel sys, ref sensors, etc.).

The car has a very consistent, violent stutter at approximately 5200 RPM....here's the catch...ONLY AT WOT. I can recreate this issue on demand in any gear (haven't tried 5th) as long as I'm at WOT as soon as it hits 5200RPM. I can drive at 90+% or any other throttle position below full and car pulls strong with no stutter right up to redline. I can run at WOT up to say 5K with no issues, and then back off the throttle slightly and continue up to redline without a problem. It seems as if the DME/KLR is kicking into protection mode at WOT/5200. A friend following me said he could see black smoke out the pipe when this stutter occurs. I can dial the boost down to very low levels (~5psi) and the car is obviously very weak on power, but the same stutter occurs so it's not simply a major overboost condition.

I've carefully checked the limit switches and full sweep test of the TPS using Clark's procedure and it checks out fine. The TPS is getting very pricey and hard to find so I'm hesitant to replace on a whim. I've also visually inspected for any vac leaks or any obvious points where unmetered air might be entering but I don't see anything obvious. I haven't yet $tarted replacing things like plugs, wires, coil, cylcing valve, bypass, DME, etc. due to this scenario occurring strictly at full throttle only.

In similar threads it seems the culprit has been the AFM or the DME/KLR. Since I'm not running an AFM it's difficult to do a simple swap test, and due to the chips I don't think I can easily swap in a borrowed DME without retrofitting an AFM in lieu of the MAF. I don't have ready access to spare DME/KLR but I'm working on locating one so I can hopefully determine if that makes a difference. I'm researching potential for a full Megasquirt overhaul, but that has to be a longer term winter project.

Does anyone have suggestions I can try, or tests I can run to try and isolate the bad component(s) without blindly spending a fortune on new bits?

Any advice is appreciated. Many thanks in advance!

GPA951s

I am very familiar with this issue, Although for me it is in the cold, (Below 50 degrees). I have been running an Authority setup for 20+ Years now and here is what I found. When the car gets up on full boost and its cold it stutters, Because Im at the limit and it senses a Knock, or over boost with super cold air. This will also happen to me when I get the car out of storage in the spring due to all the good stuff in the gas is gone.. SO....- To Test this Theory on YOUR car open the hood, Look for the "Banjo" bolt that is attached to the Intercooler pipe, Grab a 17mm wrench and remove the bolt. (dont drop the washers) Look inside the bolt and you will see what appears to be a Carburetor "JET" Remove it, (should just unscrew) put it back together and take your car out for a drive, YOU WILL see considerably less boost, but if at that time it does not "Shudder" then you need to open up that jet JUST A BIT! One drill size bigger, This will give you less boost but you will hardly notice it and your shudder should be gone.

Tom M'Guinn

If the problem always occurs right at 5200 rpm, but only on true WOT, and happens regardless of boost level, then I would suspect the DME or KLR. If I understand your post correctly, you say that the problem occurs even when you turn boost down to 5psi. If so, then I doubt the banjo bolt jet is going to help. I trust you tested the TPS signals at the DME and KLR rather than just at the sensor itself, to rule out any harness issues with those signals. Also worth checking your grounds at the bell housing. The easiest thing would be to get a known good DME and KLR to try (and just put your chips in it for testing). While trying to get those, you might try checking the blink codes at the diagnostic port to see if they tell you anything (at least one code purports to tell you when the KLR is bad). I'd also be inclined to pull the DME and KLR and look very closely for cracked solder joints. Those are known to crack and cause an infinite range of problems.

bob944

New update! Now appears to be a very different problem...

First - Thanks! to both of you for the feedback. I haven't had a chance to review and test further until tonight. Turns out I was wrong and have new info.

I pulled the banjo and there is no jet at all, but remember the PO installed a Greddy Profec S controller so the banjo is plumbed directly into the electronic boost control solenoid.

I took the car out for a careful test run tonight, hitting various RPMs in multiple gears to replicate the issue. The PO also installed a large VDO boost gauge (PSI) for easy read on track. What I learned tonight is that it seems the gauge is bad and never really reads above 5psi (not good!). I compared readings to the factory gauge which I had not paid much attention to before. Unless the chips somehow impact the factory gauge, it is reading 1.5 to 1.6 bar when the stutter seems to occur...yikes!!! I'm beginning to realize that the DME/KLR must be kicking in at what appears to be 21-23 PSI to protect. By playing with WOT from a stand still, in 2nd and 3rd gears I realized that I could indeed make the stutter occur at lesser RPMs (4K to 5K)...yet seems always when I hit around 1.5bar.

I THINK this means the DME is behaving as normal, even though I'm not sure at what boost pressure the Authority chips raise max boost level to.

So obviously I have one bogus boost gauge to fix, but I'm trying to figure out what is allowing the boost to rise so high. I've set all three levels (gain, lo boost, hi boost) on the EBC to minimum levels. I can still hit the problem regardless, it just takes a bit longer to climb to max boost at WOT.

I haven't done any further diagnostics yet. I thought when the waste gate fails it typically fails open preventing max boost level. Does it often "stick" allowing excessive boost? I obviously need to do some testing of the EBC which may have gone bad. It does still seem to have an effect (at least on gain setting which allows max boost to spool much faster) so it's hard to determine if it has completely failed only by driving. I'm starting to wonder if a Lindsey Boost Enhancer is just an easier, inexpensive route than the old EBC.

Again, any additional advice is much appreciated!

Tom M'Guinn

The factory gauge shows absolute pressure, meaning "1" on the gauge is pretty much atmospheric pressure at sea level and "2" is one BAR of actual boost. Your new info opens up the possibilities a bit, including weak ignition and over-boost protection. Lots of things in play, but I'd start by getting a good boost gauge to know how much boost you are actually running....

GPA951s

Tom is correct, and that does not seem like it would kick an overboost condition, its only like roughly 7 1/2 lbs of boost over atmosphere, not a heck of allot. I typically run 1.9 on the Gauge wich is like 13.2 with my autothority chips, and I do have the boost enhancer with the banjo bolt, all though I made my own jet so Im boosting a bit more than Originally Autothority had intended. I therefore am susceptible to Bad Gas or when it gets cold. Not knowing all the variables of your situation, have you recently got fuel? is it possible its just bad? Ive never had a wastegate "FAIL" allthough I have had a cycling valve Fail, And when they do they give little boost.

Tom M'Guinn

Yeah, I misread it to say he was getting 23psi, but now see that was probably based on a misunderstanding of the factory gauge. If it's really only making 5-7psi of boost, then that is another thing to sort out. Are you sure the boost controller isn't just set very low? Either way, stumbling on boost is frequently the result of a weak ignition (cap, rotor, plugs and wires). How old are those parts?

Perry 951

When mine would stutter/miss at full throttle many years ago, I found the cap had a few small cracks in it. Took some time to find the issue but as soon as I changed the cap, all was good.