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95 Chevy cranks excessively if sitting

n01tk

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My C3500 will not lite up quickly after sitting ( 24hrs). I installed a new fuel filter, pump, lines and TBI ( rebuilt) This truck has the long fuel pump run time (~20 secs) to ensure vapour lock doesn't occur. I installed a fuel pressure gauge and get 11 psi ( gauge cal. checked). The gauge stays steady when increasing trottle. Installed a node light on injectors and both are pulsing when truck is cranking. Looked down tbi troat of the TBI and could see fuel in intake manifold. Turned my attention to spark. Repaced coil. Checked output, above 40,000 volts. Wires, cap, plugs and rotor button replaced < 1,000 mile ago. At that time removed ign. module, cleaned grounds and replaced module with thermal conductive grease applied.
This truck runs very smoothly when it does start and goes into closed loop when temp is achieved. Checked TPS and ECT sensors, both appear to be OK. The IAC counts are 35, which indicate no vacum leaks.
I am now wondering if the fuel is being sucked into the cylinders, but my vac appears to be OK? Running out of ideas. All help greatly appreciated:mad2:
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WarWagon

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1995's always tend to have long crank times after sitting overnight. It is said the new stuff starts too quick without any oil. Starter to prime the oil system? Maybe a rumor. Just don't chase a normal thing as my 1995 350 TBI always did this long crank overnight. During the day you could bump the starter and have it running. New/rebuilt parts are not always good so never assume a new part is a good part. Always start the troubleshooting over.

Excessive cranking is likely flooded engine or a dry engine.

The injectors or other things could be flooding the engine while sitting. Open the throttle blades and look in the manifold for fuel puddles before cranking. injectors, orings, gaskets, fuel pressure regulator, etc are all things that can flood you.

This said you should be looking at the injectors for fuel coming out of them during the long crank. Many ways to loose fuel pressure overnight.

What about the fuel pump relay? Your fuel pump could be 'off' till oil pressure builds up enough to turn it on. Fuses and wiring need to be checked. First key on you should hear the pump run for a little bit.

The other thing to remember is a TBI is an electronic carburetor on a huge intake manifold. The gas vapor may condense or otherwise exit the manifold overnight. The injectors also quit dumping fuel on key off when the engine is spinning. You have to dump fuel and move the unmixed air out of the manifold - this can take a couple of spins. Port injection has less air between the cylinders and the injector. This is another factor in longer crank times.
 

n01tk

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Hey thanks for the reply. All good points. I do hear the fuel pump run when the key is turned to the on position. The fuel press comes up to 11 psi and stays there while cranking. I monitored the voltage right at the fuel pump while cranking and I get ~ 11.5 vdc, battery voltage ( new battery). This leads me to believe the relay is good. The fuel pressure doesn't stay up. As soon as I shut the pump down the return line send the fuel back to the tank and the fuel press drops to zero. From everything I read, this is normal. I pinched off the return line and the pressure stays constant all night.
I agree with what you are saying about a natural longer crank time, but it's strange some other owners state their trucks ( 5.7l TBI) fires up within half a turn. I will do the fuel checks you mentioned. 1. Watch fuel coming from injectors during initial cranking and 2. check for fuel in the manifold before cranking.
Another point I had better mention is the truck doesn't away start quickly when hot. Most times it fires right up, but occasionally it gets unperdictable. Always starts but something is not right. I am not a mechanic by trade ( industrial instrumentation) but understand electriacal/mechanical systems. I'm thinking it's the ECM not working right. This may be due to an improper input or connection. What is the ECM looking at during the crank cycle?:???:
 

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Not 100% certain on a 95, but most of teh TBI trucks don't use teh ECM for naything but firing the injectors during cranking. Until it reaches roughly 250-325 RPM's the ignition module will trigger the coil, and after 250-325 RPM's the ignition module will transfer timing control over to the ECM. I'm not sure if yours has a knock module or not as some of teh later knock modules would cause a hard start. Have you put a timing light on it to see what the timing is? I know if it isn't set right, they will not start well. I believe that truck calls for 0 at idle with the timing jumper unplugged. I normally set them to about 2 BTDC myself, but anymore tends to make them take longer to crank.
 

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I believe my engine does have the knock sensor ( just in front of starter with one wire going to it?) How do I check the knock sensor? I have set my timing at zero with the jumper unplugged, the timing is good. Thanks for the reply.
 

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I believe my engine does have the knock sensor ( just in front of starter with one wire going to it?) How do I check the knock sensor? I have set my timing at zero with the jumper unplugged, the timing is good. Thanks for the reply.
They all have a knock sensor located on the passenger side lower of teh block in the coolant drain. Some though had an extra knock module mounted up by the distributor that would control the knock retard wheras some did it in the ECM.
 

WarWagon

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An old school scope may help you with the issue. Look for a loose timing chain or a loose magnet on the distributor shaft. The 95 dist's were known to throw out an oil mist on the bottom stuff and you should look for this.

The fuel system would bleed down pressure over time for safety and changing fuel filters. Possible fuel can boil out of the injectors and take time to get liquid fuel.

The 5.7's that start quick are apples to oranges if they are not a 1995 or older TBI. My 1995 was CA emissions and I do not recall a separate knock module on it.

If it was mine I would pull the air cleaner in the AM and have a friend crank it watching for fuel at the injectors. The timing light idea is a good one. I never noticed anything weird on my 1995 with the light, but, never watched it from a cold hard start.

I also put a set of one range colder plugs in it to reduce the knock under high load freeway speed driving. Piss poor CA gas anyway with all the added O2 MTBE etc.

Yeah I agree it was a long crank time, but, I never found the issue and never had to because it always started. The dealer wasn't any help with this and the intermittent EGR issue even though they watched both happen and clear up. The EGR was the only mystery item I had with it to flop wide open at times due to a back-pressure valve failure in it. Can we say ran bad when that happened?
 

n01tk

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I will check for a knock module in behind the distributor post ( hard to see ). Warwagon what would I connect to with scope. I tried using a scope ( Fluke scopemeter) to look at the injector pulses but got garbage. Not sure if I should be using a signal for external syn. or something else I am doing wrong. What would I connect to and what are the scope settings. (in this case, maybe auto setting on scope would work ).This engine only has 51,000 mi on it and runs very smoothly when started. May not have had an easy life, bought by Utility services of Pennsyvania.
 

n01tk

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Decided to buy a can of quick start. Let vechilce sit for 24 hrs and sprayed quick start down throat of TBI. The truck turned over about one turn and fired up. Halleluiah, progress. Should have done this sooner. Pinched off vacuum line going to vapour recovery canister. Turned truck over and engine started within one turn. Will have to wait over night and and try again. This is definitely a fuel problem. Looks like I have a vacuum leak but why does the IAC show 30 counts when running?
 

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The IAC is adaptive, so it will show a certain number of counts regardless of it's actual position so long as the ECM thinks it is within range.
 

n01tk

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Don't understand what adaptive means in this instance. Does the ECM go from a bias and start counting?
 

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Don't understand what adaptive means in this instance. Does the ECM go from a bias and start counting?
The ECM uses an adaptive IAC count system. Regardless of where the IAC actually is, it will find what it considers 0 to whatever number of IAC counts. This is how it compensates for carbon buildup in the throttle body or IAC passages or what not. This way regardless of where teh IAC actually is, the IAC counts will always fall into the number the ECM's programming wants it to be in.
 

n01tk

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Built the smoke generator, could not find any vacuum leaks. Smoke white in color and hard to see. Measured cranking vacuum. 2" hg. This is low. Everything I read says 3 t0 5" when cranking. Drawing in air some where? Running vacuum is 15" hg momemtarly than it slowly moves up to 20 "hg ( 15 secs). At idle moves between 20 to 21 and back to 20 in 1 sec intervals. Any ideas?
 

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Your right, the TBI gaskets are noted for leaking but I just installed a rebuilt TBI and new gasket 3 months ago. The rebuilt unit ($410) came with 2 gaskets, I installed the one which appeared to fit best ( why 2 gaskets). I did take some propane and went all the way around the base, no change in engine speed. Discovered something interesting, when I electrically disconnect the IAC the engine starts 1/4 turn everytime? Is the IAC open when disconnected or closed. I need more informatrion on IAC control logic. Another idea is with the IAC disconnected I no longer have the leaning out effect of a additional vacuum leak? Thanks for any help.
 

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The IAC is a stepper motor. Disconnecting it sticks it in the position it was in when you pull the plug. Likely puts the ECM in a limp mode of some sort.

After you reach 45 MPH with some throttle the ECM issues 255 steps to close to the IAC completely to do a touch and learn the position of the IAC. Then it will open the IAC up to the desired position by issuing a desired number of steps open. I don't think it does this at key on. Carbon and dirt can build up on the IAC pintle and be worn off at the touch point by the touch procedure. Thus the valve can drop 200 RPM from dirt build up and stall the engine after startup. I have seen this with K&N air filters and hate them because of this. The touch point is the edge of the valve being completely closed. The touch procedure is so the computer can know what step the IAC motor is at after battery failure or other times the position is unknown - the procedure can fail to factor dirt buildup as described above.

You can remove the throttle body and possibly see the IAC position. It may close then open during cranking.

Quick and dirty is to give it 1/4 throttle and see if it starts up quick. Full throttle is clear flood mode and will crank with little fuel and not start quickly if at all.

Note the ECM stays alive and kicking after key off for 30 seconds or longer. I can read TPS on my 95 diesel after key off and get weird SES and STS light sequences, or lack of lights, if I key on during the 30 second alive time.
 

WarWagon

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Have you plugged a scanner into the computer and read out what the ECM thinks the CTS is saying for temperature? Looks like the CTS can read higher temps and lean out the engine to where it doesn't start well. The computers fail as well as wiring and CTS sensors. What I read is the circuit in the ECM can go bad thus the need for a scanner to read the CTS temp as the sensor can be fine.

Pulling the plug on a random device sends the ECM into "limp mode". All this does is tell you that the computer is going lean etc. during start for whatever reason that limp mode overcomes.

Intake manifold gaskets also can leak. Brake booster, vac lines, PCV valve...
 

n01tk

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Thanks for the reply. I have connected to the ECM with the TunerPro 5 software. The CTS shows ambient temp before starting ( after sitting over night) and follows the gauge temp up to operating temp. Goes into to closed loop. Seems to run very smoothly with a steady 20" of vac. The IAC wiring or ECM may be at fault. I do have a noid light to check for signal to the IAC. Going to connect it up but not sure what I should see? No engine light comes on when I disconnect the IAC but it may be in a limp mode. Just surprised to see how my starting issue was resolved with it disconnected. Any comments. I did find a table showing IAC position while cranking to temp. but for early GM vechicles, not sure if they have IAC. http://www.oocities.org/ecmguy.geo/bruce/prog_101.html#crank_fuel :mad2:
 

WarWagon

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Add more fuel during crank in the tune. Limp mode from open IAC wires/plug runs rich. Starting fluid, as you tested, same result. Maybe GM was too lean during cold crank on 1995's. Looks like you will need a tuned chip with this specific area addressed.
 
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