I have been chasing idle issues with my 96 LT4 since I completed the rebuild a few weeks ago.

Codes are
P100 MAF Circuit malfunction
P335 CKP Sensor "A" Circuit problem
P336 CKP Sensor "A" (18/24X) Out of range

No matter what I have done these codes are always there....ugh!!!

I have hunted and chased after vacuum leaks but I cant find anything that would cause it to idle at 1700 rpms after warm up. I did find the big leak around the intake manifold but I have since replaced that gasket and I know I have a good seal there.

MAF runs 2.44 on the scanner

Then yesterday I took it out for a quick run around the neighborhood and to test the brakes, finally got a good booster. Jumped on the brakes hard and the car stopped great but the engine died. Upon restart it backfired through the TB. Now it wont idle. I had a MAF sitting there so I put it on to see if that would change it. Nothing.

I have 30lbs Bosch injectors in it. I am going to send my old stock ones out for rebuild so I can at least get back to the stock setup.

Now it just dies and wont idle at all. Still rich! Throttle response it still good. It will run as long as I keep throttle on it. WTH!!??

Please help.

This car is going to kill me.

 

You have 30 pph injectors instead of stock ones which are 22 or 24 and you are running rich? Probably because the ECM can no longer compensate.

 

Aklim is correct. You don't need those big injectors. Go back to stock and I bet you clear up all your problems.

 

I THUNK that he has to either sit it out until his injectors come back or adjust the program in the ECM so it can compensate for the injectors.

 

Sending off the injectors for rebuild tomorrow. I will sit it out until they come back. I can also do some circuit testing on the MAF and the CPK and make sure they are getting the correct voltage.

But still don't know why it would run for a few weeks at high idle and then now all of sudden it wont idle at all. That's a weird one.

 

Spark plugs could be fouled from the extra fuel. Haven't given it much thought as what vacuum leak does on a MAF car...I'm running SD.

 

Who's doing it?

 

Fuel Injector Connection. I spoke with them a few days ago and he said he could get them setup no problem.

 

Stock LT4 injectors are 28# The PCM will easily compensate during Closed Loop operation for the larger 30# injectors. The engine will run a bit rich at start-up and during WOT but nothing outrageous and it's not going to cause the codes the OP has.

You said you just finished a rebuild. Did you pinch any of the Crankshaft Position Sensor (CPS) wires? Have you performed the troubleshooting for P335 and P336? Have you checked to see if there is a shared ground that causes P100 for the MAF and the two CKP codes?

FWIW I had the exact same two CKP codes on my '96 GS....the sensor was bad. Not sure how/why....as I recall it's just a hall effect sensor....but replaced it with a new one and 10 years later when I sold the car all was still fine.

 

IDK. IF it is at 28 then it should be fine. Still, I'd wait till I get the old rebuilt injectors back and see if that makes a difference.

 

I have not tested the wires for CKP codes yet. That is next on my list. I thought I would start with the MAF.

I was using my test light last night to try to verify the ground at the MAF. I didn't have much luck. First thing I did was verify ground from the battery to the engine. I have good ground there. I pulled back the wire covering all the way from the MAF plug to where the wires join the rest of the group. There was no brake in the line or damage to the coatings that I could see. As far as I could tell the wires are green orange and red. They are so dang dirty, even after cleaning. Ha! Using my test light I could never get a it to light up for a ground connection putting the probe into the plug in the different wires. I was able to verify the ground on the EVAP purge valve plug. I think it is on the same common ground. I do have a new plug with pretty long pigtails for the MAF. If it is a ground issue I could always hook up the two wires, signal and power and then run a separate ground from the plug to the block to verify...

I also ran scanner in test mode on the IAC I can change the throttle up and down with the scanner and it should pulse the test light on the IAC plug and all it would do is stay solid. Not sure about that thing either. Or if my test is valid. Since the car wont idle testing is difficult. It will run with my foot resting on the throttle but it so rich now it cant run.

I bought the current injectors used on the site here and I wonder if they are leaking. Seller said they sat for a month or so after being used. I did get the old ones sent for rebuild yesterday so when they come back I can remove that part of the equation. Until then I can continue to try to verify connectivity on the MAF and the CKP.

 

Keep in mind the CPS has nothing to due with fueling or timing on the C4. As such, you may have multiple issues.

Also, just looking at data on a scanner really doesn't tell you much. It's kind of like looking via a soda straw. Need to plot and look at it graphically.

Are you sure the Opti is on correctly? Although more difficult to do than the '92-'94 units, the '95/'96 Opti can be installed incorrectly. This often happens when people seat it by tightening the bolts rather than seating it by hand.

Certainly curious to see what happens with the injectors. You can check for leakage by putting a pressure gauge on the fuel rail and turning the key to "run". The pressure should hold steady and slowly bleed down over 30+ minutes. You can also lift the fuel rails with the injectors still attached out of the intake bungs and then turn the key to "run". That will power the fuel pump for 2 seconds and if the injectors are leaking you'll get the associated puddle or drips of fuel.

 

I put the opti on by hand then tightened it down. It took several attempts but I made sure it was seated by hand. Also when the car is running it runs good, well as goo as expected with the rich situation, and has good power. There is no miss or stumble. It also starts up very easy when cold. When hot it takes some turning on the starter to get it come to life.

Work on the car has slowed alot due to school start up and kids stuff every evening now. But I hope to get to it this evening.

 

Update:
Got some time with the car today.

Went to the FSM and started looking at the procedures to test the MAF circuit again. I also had the page for the actual wiring diagram open, this shows the colors of the wires a well. I tested the ground first on the MAF plug. Good ground using a test light. I then pulled the crank position sensor plug and tested it for ground. Good ground there as well. They share G106 which ties into the big ground on the side of the motor. With KOEO now I go back to the MAF plug and I am not getting a reading of 5v on the signal circuit. Its reading 0 on my meter. I get 12v on the power line. But that is happening on plug wire A when it should be on plug wire C. I am completely confused at this time. Staring at the car I think WTH and test the what would be the signal wire on the CKP plug and low and behold 4.99v. WHAT?? Pull back the wire loom on both plugs and clean with some solvent and that reveals to me the colors on the CKP are the colors for the MAF. Holy hell I had them switched... These two wires are about the same length and come out of the same group at the same spot. I took all of the wires off of the motor about 2 yeas ago thinking I would get the rebuild started back then but then had a huge delay. When I got it all back together I had switched these two. But I was a little confused as the colors on what should be the CKP aren't the same as the manual and I can tell its the original plug and wires, no cuts or splices. I plug it the CKP sensor anyway and proceed. Go to the car and check the codes with KOEO and still have the same 3 codes I have always had. Clear them out and check to make sure they are clear. MOBIUS says ***No Codes to display*** Moment of truth. Start the car and BOOM just like that no Check Engine Light at this time! Do I feel a little dumb? Sure. Am super pumped about getting rid of CEL? HELL YEAH! That feels like a big step.

Did it idle? NO. Injectors are at the shop for rebuild. In the mean time I will clean the plugs. Check the IAC as well.

Progress can be slow sometimes but this was a big win, even if it was something so simple as a switched wire.

Once the injectors come back if it still wont run right I am thinking about a new IAC sensor and sending the PCM off to PCMForless and have them set the tune to my new cam. That way I am starting with a fresh tune and fresh motor.

 

You have a different cam?! <smh> That little tidbit is kind of important. Anything much larger....particularly regarding duration....and you need a tune at idle.

Like I said....put a fuel pressure gauge on the rail and power it up. If it bleeds off, you have a leak. That could be the injectors, the Fuel Pressure Regulator (FPR), or the pump. Told you how to check the injectors. FPR is easy...pull the vacuum line off of it and if there's fuel in it, it's leaking. That narrows any leak to the fuel pump. Having said all that, you usually have hard starting if any of those three are leaking. If the fuel pressure doesn't bleed down, injectors are not your problem. Btw...as I recall, the Bosch injectors are rated at a different fuel pressure than the GM injectors. Did you tweak the fuel pressure as well with an adjustable FPR?

Now that we know you swapped cams (and what else?), I'd lean towards the tune.

 

The cam I have is GM Performance LT4 HotCam, https://www.summitracing.com/parts/n...+5e207e24bb901

Cam Style:Hydraulic roller tappet

Basic Operating RPM Range:Idle-6,000

Intake Duration at 050 inch Lift:218

Exhaust Duration at 050 inch Lift:228

Duration at 050 inch Lift:218 int./228 exh.

Intake Valve Lift with Factory Rocker Arm Ratio:0.525 in.

Exhaust Valve Lift with Factory Rocker Arm Ratio:0.525 in.

Valve Lift with Factory Rocker Arm Ratio:0.525 int./0.525 exh.

Lobe Separation (degrees):112

Computer-Controlled Compatible: Yes

It is supposed to run with stock tune. I have no idea if this is the stock tune though because when I got the car it was left for dead and I had no contact with the previous owner. Flying blind there.

I have been told by FIC that my injectors are 30lbs. Original LT4 are 28lbs. I checked the ohm resistance for both my originals and the Bosch and they are both equal at 12.5 IIRC, I do know they are equal. There are some that are low resistance and some that are high resistance. I wanted to make sure I was dealing with the same thing.

The car does have a new EBAY fuel tank, a new AC Delco Sending Unit and Fuel pump, a new FRAM inline fuel filter, a new FPR from carparts.com. I thoroughly cleaned out the fuel lines before installing everything as well. Today I will be checking the FPR for leaks and get a gauge for the fuel pressure. I did not adjust the FPR.

 

At this point, I would get the car down to spec and get a dyno tune before chasing anything else.

 

That's an old old old cam. Been around for at least 25 years and maybe closer to 30. "Computer Controlled Compatible" only means you can run it in a car with an ECM (pre-'95) or PCM ('95 & '96). It doesn't mean it works well with the stock tune. The cam is why you're running rich and it has a somewhat choppy idle. I had that cam in one of my '96 GS engine builds before I built the 383 for it. The idle was much happier with a ~100 rpm bump. The fueling tables needed to be cleaned up across the rpm & load bands and the timing needed to be tweaked. You'll also find it likes to rev a bit higher than the stock cam. I bumped my redline to 6700 or 6800 and ultimately swapped to better valve springs when I did the cylinder heads. So yes, it'll run with the stock tune....but it's not optimized. As I recall, when I tuned my engine after *just* the cam install, I netted ~40hp at the rear wheels, so ~50hp +/- at the crank. At the time I used "LT1 Edit for OBD II"....unsure if that package is even sold anymore. Yes, it can be used on the LT4.

Lastly....and my memory is super fuzzy on this point....but I recall the stock injectors at the stock fuel pressure being at 100% duty cycle at certain rpm points. The hatchet method to address this was to bump the fuel pressure while scalpel method is to add larger injectors. Both require a re-tune. Ford Motorsport injectors used to be the popular swap.

Going back to the stock injectors may lean it out a bit, help at least make it drivable, and keep it from dying at idle. I'm also guessing that after it goes into Closed Loop, it idles a bit cleaner, so the richness is at startup.

Here's the old hot rod and the 383 build

 

Tiny nitpick, but PCM started in 94.

And no, LT1 Edit is no longer sold. Good riddance to bad rubbish on that--it's $450 locked to a single PCM, and an additional $50 just to actually have gotten software updates after purchase!! That makes JET DST's $400 actually look like a good deal. I understand it was "back in the day," but man...I really hate predatory software licenses like those. It seriously irks me.

 

Update:

FIC called me and said my old injectors were frozen solid and were not going to be able to be rebuilt. He sold me a set of new Bosch injectors with the proper flow rating for my car. Approximately the same price for what the rebuild would have been. He even machined in the groove so I could use the stock retaining clips. I received them and they are installed.
At the same time Keith from PCMforless set me up with a new PCM. Since my car will be registered as an antique in TX in about 2 months he turned off a few things I will no longer need, EGR, EVAP, Skip Shift, Air Pump, Upper O2 sensors and setup the rest up per my cam and 6speed.

I got both the PCM and the injectors installed and the car runs good. The PCM is plug and play no need for me to do anything to it. No more rich smell at start up. Plugs are clean. Codes are gone and there is no check engine light!!!! That is huge!!! BUT - Idle is still high.... ugh.

I decided to replace the IAC. I bought an AC Delco unit. Installed it per the manual. Now it tries to idle down to the 850 rpm desired idle and then dies. It wants to die whenever it's under 1000. This motor should still pull enough vacuum to easily idle at 850. Today when it stops raining I plan to put a vacuum gauge on it, once again and try to figure out if there is a leak or if I need to mechanically adjust the idle set screw on the side of the throttle body. Will pull the throttle body a see if there is any light getting through when the blades are closed per the current set screw setting. I don't think that is the problem though....

Wish me luck.