Evan70

Well, I initially thought it was overboost protection causing the cut off under full throttle, but now I'm not so sure. Here's the symptoms:
On full throttle runs in 3rd gear or higher, after a second or two of full boost I loose all power (engine still runs). After that, the car doesn't run well at all unless I turn it off and restart it (it will lug and miss really bad if I don't restart). I had thought it was an overboost issue as the car has a MBC installed from a previous owner and would easily hit 2 bar on the stock gauge. Since nothing I did with the MBC ever seemed to make a difference, I ordered an electonic boost controller (SBC-iD) for better control and to be able to see exactly how much boost I was getting. So after setting it up last night I started doing some test runs, and I still get the same issue even if I limit boost to 11psi. What's wierd is that if I don't go full throttle (but say 90% instead) I can run 14psi just fine (haven't tried higher than that yet). Also, from the looks of the fuel curve on a dyno I just did (could only get a run in 3rd due to this same problem occurring every time in 4th), it looks like the car may have Autothority chips installed (very rich mid range).

I recently replaced the TPS when I was having stumbling problems, could this be causing the problem if its not set exactly right? The actual cause of the stumbling turned out to be the airflow meter, which has been replaced as well. I didn't have this problem nearly as much when I first bought the car last month (although it did happen occassionally even before the TPS and airflow meter replacement). Other than this issue the car runs great. Like, I said above, as long as I don't go full throttle I can easily pull 14psi from 3K to redline with no problems. In 1st and 2nd I can go full throttle and never have this issue, which made me think it was overboost, but I just don't know now. Somebody please help.

hosrom_951

What chips are you running?

Do you have an EBC or MBC? did you delete the cycling valve?

Is your TPS aligned properly (you should hear a faint click coming from the throttle body as you open/close the throttle) but i suspect that is not your problem........

Evan70

No idea what chips are in the car. I did have a Lindsey MBC, but I've now switched to an EBC (SBC-iD). The problem occured under the MBC, and is still happening with the EBC as well (even when set as low as 11psi). The car also has a Lindsey DP wastegate. As to the cycling valve, I'm not sure. I just got the car last month. Where do I check for that? The TPS seems to be aligned correctly (I get a click when I manually open/close the throttle body).

hosrom_951

TPS is aligned correctly, and probably isn't the fault there since nothing happens on 1st and 2nd gears

There is more load on the engine on 3rd and 4th gears, but what happened with me is that the ignition coil was failing and gives me an overboost/fuel cut-out situation........

Pull out the coil plug and look inside for any corrossion, also check the cap and rotor.....

The CV is located underneath the intake manifold, you could see it and unplug it, but you cannot remove it unless the intake manifold is off......



Picture courtesy of clarks-garage

Evan70

I've recently changed the plugs, wires and distributor cap (I've got a new rotor to put on, but honestly the one that was on there looked fine). I'll check the coil and see how it looks, but I'm guessing it's stock (68K miles on the car) so that very well could be it. Please excuse the newbie question, but what does removing/unplugging the cycling valve do?

hosrom_951

It determins the amount of boost available.........

dand86951

Since you say you can get a smooth pull when using partial throttle even up to 14psi but can't when using full throttle I am going to suggest something is wrong with either the fuel mapping or timing control. At partial throttle your throttle position is telling the computer you don't need full rich mapping so you get a leaner mixture than you would at full throttle. The quick application of full throttle sends the mapping to full rich throughout the rpm range if I recall correctly. If it goes too rich you will get stumbling and power will come off, you could also be overcoming the coil and sparkplugs ability to fire. Don't know why after a full throttle run the car would continue to run poorly until you restart though.

If full throttle doesn't send the mixture over rich then you may be getting into detonation and the knock sensors could be cutting the engine back, however if it runs good at partial I don't see how that could be unless the chips are really off. If they are Autothority they wouldn't do that unless you are running non stock fuel pressure and or non stock injectors.

F18Rep

Well, except for the continues-to-run-badly part, mine is doing the same thing. Last night I finally devised an adapter that lets me inject compressed air into the J-pipe. Then went at all the suspect areas with a squirt bottle and some soapy water. Like others have said, I found big leaks but the locations surprised me some - at my intake gaskets and the throttle body. Also can hear a pretty good hissing from under the intake, so that will come back off. Lessons continue....bruce

Tom M'Guinn

Have you changed your cap, rotor, plugs and wires yet? The fact that it only happens in 3rd and higher suggest to me it is a load-based problem, and weak ignition is a frequent cause of WOT problems in higher gears. The continues-to-run-badly issue, however, is a head scratcher...

Evan70

Well, when I did get it to make a full run on the dyno (in 3rd), it was VERY rich in the midrange. Here's a link:
http://www.lt1.net/images/951/dynobase.jpg
From 3800-4600 the A/F is completely off the chart (less than 10:1), and it doesn't come back to 12:1 until 5700rpm. From everything I've seen this looks like a fuel curve from Autothority chips, which from talking with Autothority last month (I had a spare set I was thinking of putting in that the previous owner bought but never installed), they said NOT to use a boost controller with their chips.

Anyway, I just ordered a new coil and will put that in as soon as it gets here. I've also been planing on getting new chips from Vitesse or Guru, and will probably go ahead and do that too even if the coil doesn't fix the problem. I'll also start doing a thorough check for leaks.

Evan70

As I stated above, I just changed the wires, plugs, and cap. I haven't changed the rotor (the old one was really stuck on and looked fine anyway), and I haven't done the coil (but ones on the way).

Evan70

Well, it's still doing it after replacing the coil. Also I've narrowed it down to specifically in the 3,000 to 4,000 rpm range under full (or almost full) throttle in 3rd gear or higher. If I go full throttle from 4,000rpm it doesn't cut out. Initially when I got the car a little over a month ago, I didn't notice the problem very often (but then I'm not sure I was meeting the conditions either), but now it does it consistently. This is really becoming very irritating, and I'm getting ready to take it into the shop as I'm about out of ideas. At this point I'm pretty much ruling out the chips and A/F as a problem, since it runs fine if I start at 4K and the A/F doesn't go really rich until 3800.

At this point the following have all been changed within the past month:
coil
changed from MBC to EBC (and have tried running as low as 11psi)
airflow meter (used)
plugs & wires
distributor cap

Could it possibly be the airflow meter having issues. I know it maxes out and switches to a table in the computer somewhere around 3500rpm. Could there be a problem with the upper end of the afm. It really seems like this issue has been happening a lot more since I changed the unit out.

F18Rep

OK, this is a shot in the dark, but there is a pot in the TPS - could it be that the pot arm is lifting off of the resister winding at a particular spot (throttle angle)? You can check it pretty easy with a meter - just a thought...bruce

Evan70

Sorry, I left that off my list. I've already replaced the TPS as well with a brand new one. I talked to my mechanic (Tom Charlesworth) and am going to run by at lunch for him to take a look at it. He's got a couple of ideas on what it might be.

Evan70

Ok, after running by Tom's at lunch he said it's definitely something to do with the overboost protection. We also found that the stock cycling valve has been bypassed too. He suggested hooking the stock cycling valve back up and see if that stops the problem, but he also thinks it may be the chips in the car (probably Autothority). Anyway, I'll hook the cycling valve back up tonight and see if that fixes the problem, but I've already been planning on new chips, so I guess I need to get off my lazy butt and get some.