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LT4 Injector Pulse Width Question

1996 LT4 Topic

Guitared

Member
Joined
Aug 22, 2007
Messages
18
Location
Windsor, CO
Corvette
1996 Black LT4
I would appreciate any help. "The Bullet" has been sitting in my garage all summer. Withdrawal symptoms have become unbearable. I need a fix for me and my Vette.

I've been troubleshooting my LT4 for over two months, off and on. The problem is the fuel mixture is far too rich. It smokes and smells of gas and runs terrible. The only way to start it is to push accelerator pedal to almost WOT. The only DTC code is P0300 (random misfire - I knew that). It runs too rich at initial starting (open loop) and after it has warmed up (closed loop). I've clean and tested the fuel injectors. I'm confident they are working properly. I've replaced the fuel pump, ignition coil and spark plugs. The spark plugs, all 8, were heavy carbon fouled but not wet. The fuel pressure is OK at about 45 PSI.

I measured the injector pulse width using a current probe and oscilloscope at idle, open loop. The PW is approximately 10mS on all eight cylinders. I measured from the beginning of current flow to the termination of current flow. The actual time the injector was delivering fuel is about 1.0mS less considering the time needed to open the injector. I can see this as a bump on the current waveform. Researching online, it appears the PW should be on the order of 2.5 to 3.5 mS at idle. Having said that I know this value is for ordinary cars, not necessarily for LT4s.

I have two questions:
1) Does anyone know what I should expect for the PW at idle?
2) Assuming the PW is spot on and I move my troubleshooting effort to ignition, is there an easy way to determine if adequate spark is being delivered. Looking at the effort and cost of replacing the Opti-spark, I'm hoping I can troubleshoot it without just replacing it.

Thanks in advance for your help. Also, if it turns out the PW is too wide, where would I look for the trouble that would affect closed and open loop control?
 
Assuming 600RPM idle and firing the injector every other engine revolution...10mS comes to a 5% duty cycle. Doesn't sound unreasonable? (I'm no LT4 expert though)

Was that FP measured while the engine was running?

If the duty cycle or fuel pressure aren't causing it...only other thing that effects amount of fuel delivered is the flow rate of the injector. Were they replaced at some point? Maybe got the wrong flow rate?
 
Do you have a scanner that will freeze frame the PCM's info and print it out?

Maybe you could find someone who could do that for you.

With that info we could steer you in the right direction.

SAVE THE :w
 
Have you checked your 02 sensor? If that's bad, I think it could effect your fuel mixture.
 
1) is this engine modified? If so, what mods have been made.
2) Once the system goes closed, does it run rich all the time or just at idle and part throttle? Also, how are you telling when the system is in closed loop?
3) Have you run the fuel pressure checks in the Service Manual? If not, now's the time to do that. Also, the answer to that question is not "The fuel pressure is 45 psi". The answer is: what happens to the pressure when you run the checks in the FSM.
4) Are there any other codes set other than DTC0300?
 
Thanks for all the help and suggestions. This is a tough nut to crack. The fuel is so rich, there are half-dollar size black water/carbon puddles below my exaust pipes. The problem appeared quickly. I was out driving with my son looking for a car to buy him. It was over 90° that day. We got back in the car and it was hard to start. I had to press on the gas to get it started which is unusual. It had ran fine all day although the engine ran hot, around 235°. It has got progressively worse since then. I must admit I'm not excluding the possibility that my working on the car could be making things worse.

The engine is completely stock. The problem is present on open loop and closed loop. I'm thinking the O2 sensors are not the problem because they are not used in open loop. I have a code reader with freeze frame. The data from yesterday is (after resetting the DTC and moving the car to the driveway):

DTC P0300 (no other codes present)
Fuel Sys 1 OL
Fuel Sys 2 OL
Calc Load (%) 5.49
ECT (°C) 31
STFT B1 (%) 0.00 (this has been as high as -50)
LTFT B1 (%) -1.56
STFT B2 (%) 0.00 (this has been as high as -50)
LTFT B2 (%) -1.56
MAP(kPa) 28.00
Eng RPM 1206
Veh Speed(km/hr) 7
MAF(g/s) 12.00
TPS(%) 0.00

In the past, I have allowed it to run long enough for it to go into closed loop with similar freeze-frame data. The only other DTC's I ever got were a P0305 (cylinder 5 misfire) or P0306 but mostly P0300.

Hib, I'm totalling on-board with were you're going with your questions about running the FSM fuel system diagnostics. I have done some. I'll tell you the fuel pressure is about 45 PSI during the 2 seconds the pump is on before starting. It then drops off to about 20 PSI and stays there for some time after the pump stops. It's my understanding this is not acceptable. The FSM says "The fuel pressure may vary slightly when the fuel pump stops running. After the fuel pump stops running the fuel presuure should stabilize and remain constant." I have a couple of problems with this test.

I've had some trouble with my fuel pressure gauge ( brand-new from Checker). The pressure relief valve is a little leaky at times (and sticky - I have to flick it to get it closed) and I don't have much faith in it. Perhaps you can recommend a better gauge. Also, the FSM diagnostic test indicates the need to insert fuel pipe shut-off adapters (GM J 37287) to isolate the fuel pressure drop-off problem. If the pressure loss is on the fuel pump side, then it is not causing my rich fuel problem. On the other hand, if the injectors are leaking it is directly related to my problem. Problem for me is - I don't know where I can buy a GM J 37287 to complete this test. Any suggestions would be of great help.

I'm going to take a look at the ignition side of things today as well as the EGR.

I appreciate everyones help with this. Thanks!

A recent Harvard study found Americans walk about 900 miles a year, on average. The American Medical Association says Americans drink about 22 gallons of alcohol a year. That means we get around 41 miles per gallon.

Makes you kind of proud to be an American don't it! :D
 
Forget all this stuff about pulse width.

The first symptom you need to work on is the fuel pressure problem. If the pressure is 45 psi with the pump running then drops to 20 when the pump quits, you have a big leak somewhere. The combination of low fuel pressure and running rich has me thinking
you've got one or more leaking injectors.

As for the Checker gauge, if it is not working properly, I'd return it. The gauge I use is part of Kent-Moore's Fuel Injection testing kit, which also includes J37287. The darn things are bullet-proof.

Confronted with not having all the fuel system diagnostic equipment, I think I'd pull all 8 injectors and send them to a facility which cleans and flows injectors.

My guess is you're going to need to fix the fuel pressure problem before you can effectively address the P0300 codes.
 
You're right! They are in-route to RC Fuel Injection. I'll let you know how it went next week.

Thanks for your help.
 
RC Engineering...good choice.

They've done my fuel injector cleaning and flow testing for many years.

It just might be that your misfire codes will disappear, if the problem turns out to be leaking injectors and you replace the leakers.
 
My injectors came back yesterday from RC Engineering. Two of the injectors tested as "dripping" for the pattern test before cleaning. All tested excellent after cleaning. I installed them and tested the fuel pressure without starting. The pressure went to about 45 then dropped to about 40-41 when the pump stopped. Looked good. I fired her up and she ran great for minute or two then the rough idle came back along with the smoke. I retested the pressure and sure enough, it was leaking again. The pressure fell from 45 to 20 when the pump stopped. My son happen to be in the garage when I grumped about it leaking again. He said he knew because he could see it. Whaaaaat?

Turns out the fuel pressure regulator failed and was suppling fuel through the vacuum hose. This regulator was new last summer. I had changed it last year trying to fix what turned out to be a fuel pump problem. When the rich fuel problem first reared it's ugly head I tried the old regulator with no help so I put the new one back thinking it probably has more years left on it. Wrong! My guess at this point is the injectors were possibly the original problem and in the course of troubleshooting, the regulator also failed. Who knows for sure.

I put the old regulator back in and she's running great now. I've re-terrorized my little town. I think they missed me. Hib, thanks so much for your help with this. You were right on target! Someone once told me "listen to the parts and they will tell you what's wrong". Good advice from you. Much appreciated.

Sorry about the long winded followup. I think this forum thing works great and maybe someone else with a similar problem will benefit from my long explaination.

Phil
 

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