I have developed a problem with my 90 Vert with a 383 running 11 psi of boost. The engine was built from the start for FI so it is low compression (8.4:1), ProCharger P600B, forged internals, 60 lb/hr injectors, Crane Fireball Ignition, water/alky injection etc. The car was running good at 8 psi so I changed the pulley to bump up the boost to 11 psi. The car seemed to run well at 11 psi but i never went pass 8 psi due to not having had it dyno-tuned to the higher boost level.

I wound up taking the car to True Street, a local dyno-tuner in the Dallas, Tx area. During the dyno runs the car would stumble and basically flat line around 4500 rpm, (no I am not running the stock intake). It actually developed 40 RWHP less than it did when I was running 8 psi. The other casualty of this session was the automatic transmission TC lock-up command. The car would lock up the TC before the dyno-tune but did not lock-up as soon as I left the shop. I now have the ALDL pins A and F jumpered and switched so I can manually command the lock-up which works just fine but is a pain to deal with. I am getting off track however.

So, here is what I have found. If I push the throttle while watching the boost gage and only take the engine to the point of 0 psi, the engine spools up smoothly and will hold 5500 rpm without a hitch. The second the manifold pressure goes positive, the power drops off and the engine stumbles. It feels as though either the fuel or spark is being affected. The car does not buck or backfire, it just goes nowhere and the expected power/rpm rise does not occur. I put a direct reading fuel pressure gage on the car and under throttle the fuel pressure would stay steady around 42-43 psi. At idle it would drop down to around 38 psi. I also replaced both the 1 bar and 2 bar MAP sensors but the symptoms persist.

I seem to have no other problems other than the no TC lock-up command from the ECM and the stumble under boost. One other thing, on the TC lock-up issue I have gone into the current tune and checked to see if the tuner changed the lock-up perimeters, which he did not and I have installed an earlier tune when the TC was locking up on ECM command. No change with the earlier tune. However, it locks and stays locked when I use my switch.

Anyhow, if you have hung in there long enough to read this entire post, i appreciate it. I would also appreciate any ideas on where to start my gremlin hunt.

Thanks,
90Indy

 

Did you capture any data logs via a scan tool, and/or did the dyno shop measure AFR / boost during the pulls?

Sounds like it could be one of several things:

1. Slipping pulley (did you see the boost rise?)
2. Fuel pressure not rising with boost (do you have an FMU?)
3. Poor fuel pump ground (losing voltage, going lean)
4. Spark plugs over-gapped (old?)
5. Other ignition issue maybe (I had a similar problem when the opti crapped out on my 92, but I guess you have a regular distributor)
6. Plugged Cats
7. Bad gas
8. Arcing spark leads

Probably need to start from the basics and work through it..

 

The first thing I would do is check the plumbing. If a clamp has come loose and a connector is open, there goes 100% of your boost.

I experienced this once.

 

I have to agree with DJXIB
Did they have an exhaust gas analyzer it may be leaning out at 4,500 rpm
Perhaps the ecm picked up knocking? maybe bad fuel? there could be your stumbling. I mean you have big enough injectors as long as you know the fuel pressure was there at those rpm's, you may be confident in the fuel system. You are pretty much guessing at causes without an analyzer to pick up on what is actually happening. With that compression ratio you may want to go an atmosphere of boost

Anyway sounds like a simple issue.
Good luck

 

I am thinking along those same lines. It seems to me, the 2 bar MAP signal is not being received, therefore no increase in fueling. However, my wife and I are celebrating our wedding anniversary this weekend so you know where this issue rates in my weekend plans.

I appreciate the posts and ideas. When I get it sorted out I will let you know what I found.

90Indy

 

Any chance you have some collapsing ductwork feeding the blower that could choke the airflow?

What are your pulley diameters? Any chance the impeller tip speed is going supersonic above 4500 rpm?

Please explain your setup for our understanding, especially from the controller perspective. Are you running both 1 and 2 bar MAP sensors somehow simultaneously? Confused on this point.

 

As far as collapsing duct work on my set up it can't happen. The blower output goes directly into a 3.00"diameter aluminum tube bend which is welded to an aluminum Vortec plenum. The plenum is connected to the throttle body via a thick robber hose. The gap between the plenum and the throttle body is about 1/4". This same gap exists between the blower output end and the tube bend. I use a water/alky injection system and I do not have a installed. The exact pulley diameter I do not know off the top of my head. I ordered from ProCharger specifically for the P600B.

I am both the 1 and 2 bar MAPs and have been for about 2 years. This problem is a new one as the car has been running good since I put fit al together.

Since I am sitting at a winery supposedly enjoying my anniversary I cannot be any more specific. However, I would appreciate it if you have more questions you ask them and I will be more than happy to give the details you need.

Thanks for the reply,

90Indy

 

As far as collapsing duct work on my set up it can't happen. The blower output goes directly into a 3.00"diameter aluminum tube bend which is welded to an aluminum Vortec plenum. The plenum is connected to the throttle body via a thick rubber hose. The gap between the plenum and the throttle body is about 1/4". This same gap exists between the blower output end and the tube bend. I use a water/alky injection system and I do not have an intercooler installed. The exact pulley diameter I do not know off the top of my head. I ordered from ProCharger specifically for the P600B and 11 psi.

I am running both the 1 and 2 bar MAPs and have been for about 2 years. This problem is a new one as the car has been running good since I put it all together.

Since I am sitting at a winery supposedly enjoying my anniversary I cannot be any more specific. However, I would appreciate it if you have more questions you ask them and I will be more than happy to give the details you need.

Thanks for the reply,

90Indy

 

Hi Indy !

The problem is the new pulley - obviously ! The tune you have expects boost at a given RPM and without it you have way too much fuel so engine goes flat. That's the problem with a S/D and 1 BAR MAP (His 2 BAR is only for the water/alky) tune. Works perfect as long as boost matches RPM.

Either pulley spun on blower or belt is slipping badly (or blower itself but I doubt). Put old pulley back and new belt and let us know.

BR
greg

 

Your fuel pressure should go up with boost or it will go lean. IE if 42 psi wot and no boost you should have 50 psi with 8 lbs of boost.

 

Check the vaccum hose to the fuel pressure regulator.