1995-Chevy 5.7 TBI-Long Crank cold

Chasing an issue in a boat-1995 Mastercraft with Chevy 350 TBI-HEI ignition etc. Really no different than a 1995 Silverado pickup.

My problem is it is has to crank for a while to start when cold(5-10 seconds), and sometimes after a heat soak it will not start. When the heat soak no start condition occurs I can spray a bit of carb cleaner and it kicks off without an issued. Have verified both ignition pulse and spark during these conditions. Have also verified fuel pressure is there during cranking. Once running it runs fine, and will start immediately thereafter. Issue is only occuring on a cold engine or heat soaked (say an hour or so)

I think the issue is related to the ECT (two wire NTC thermistor). It is Pre OBD II so I can’t plug in a scanner to see what temperature or voltage the engine is sensing. I have checked for codes and it is just giving me code 12 so no codes.

Over the weekend ambient temperature was in the 40’s, and the engine had not been run in over 12 hours. The thermistor read 3,600 OHM’s and based on the chart that would be 60 degree(not surprising as the engine is in an insulated doghouse). Then I did KOEO and tested voltage at the thermistor and it read 2.5 Volts(the previous day it read 1.7 volts when the engine was at operating temperature of 140 degrees). The connector that goes on the thermistor did appear to have been front probed before expanding the connector, and there was rust/corrosion in the terminal from the salt water.

Trying not to be a parts changer and just throwing a new connector at it-Is there a way to tell what the voltage output should be at that resistance? Using Kirchoff’s law that I learned from watching the SD Premium channel if I am reading 3,600 OHM’s and a voltage of 2.5V on a 5V circuit that must mean that the resistance of the resistor in the computer would be 3,600 OHM's. In the Section 6 videos about thermistors Paul notes that the Snap On scan tools give you voltages-Does anyone know if the 2.5 Volts sounds correct for an engine with 60 degree water temperature? I am hoping not as this might be why I am having some hard start issues.


I have tested the ICM and it is good, new plugs, wires, cap, rotor, and it is putting out a good spark when it won’t start. Again have confirmed injector pulse as well. Any thoughts on the above or other items to check? A few specifics-it is a MAP engine, timing is set at 10 degrees BTDC per the book, throttle body is a Rochester 2 barrel, system fuel pressure is 30 PSI per the book(they used 454 throttle bodies on the 350 in this year of my boat), stepper type IAC motor, TPS is same as on a Chevrolet truck. Engine turns over fine-battery is strong and cable connections are good. Battery cables are new in fact.

Any help would be greatly appreciated.

How quickly is the fuel pressure falling off after you stop the engine ? and how quickly does pressure build back up after a cold start/heat soak ?
My thinking is the non-return valve in the pump might be leaking pressure back to the tank.

Fuel pressure does drop off fairly quickly with key off. I have been told that was normal as it is bleeding back through the regulator. With key on pressure builds back immediately
I have waited for pressure to build before cranking and long crank still occurs.

Have you checked for a vacuum leak?
Is fuel fresh?

Fuel is fresh but have not checked for vacuum leak. I will have to try that out. Really no vacuum hoses other than the one going to the MAP, but could be an intake leak. Do you prefer propane or water bottle? Thanks for taking the time to respond

Any thoughts on the coolant temp sensor voltage I was reading?

If you are concerned about temp sensor then you can try to bypass it with a regular resistor and watch the result try something like 5.1kOhm or 10k resistor. Injector balance test? (Was it a 3,600 Ohm or a 3600 Ohm = 3,6kOhm reading?)

My concern is with the harness and the voltage I am reading when I check it. Sensor is working properly-I just think it may be lying with the harness connected

Lets go through what we seem to know.
(1) It looks like the hot start is difficulty is because of a lean condition.
(2) The CTS itself appears to be within normal specs.

Unknown.
(1) Is the poor cold start because of a lean condition/lack of cold start enrichment?
(2) Is there an issue in the CTS wiring or PCM?
(3) Is the engine running with a normal airfilter ?
Anyone could do some CTS voltage readings from a similar engine it would be useful.

Connectors can be cleaned of green corrosion by soaking them in vinegar for a couple of hours -- disconnect the PCM and battery first.
Afterwards wash well with boiled water, then wash with a dilute baking soda solution and wash again with boiled water then spray with switch/connector cleaner-lubricant.

Thanks Andy. Yes I agree the CTS is working as it should. Will try the cleaning technique you noted below. If anyone has a Snap On Verus and could check under competent test for the CTS I would love the know what temperature it says equates to 2.5 V.

Lean condition-will have to check that out. The throttle body gasket is new as I had taken it off to clean it. If there is an intake leak it may just be st the intake manifold. I always had in my mind vacuum leaks would show when the engine was cold so I hadn't thought to chase that.
As far as wiring integrity to PCM I have confirmed it is good. PCM not sure. This would be pretty simple if I could just plug in a scanner and see what the computer is seeing, but this is pre ODB-2 by one year

The engine uses a flame arrestor instead of an air filter. Basically a metal mesh. I have cleaned it thoroughly

AUJeeper wrote: Thanks Andy. Yes I agree the CTS is working as it should. Will try the cleaning technique you noted below. If anyone has a Snap On Verus and could check under competent test for the CTS I would love the know what temperature it says equates to 2.5 V.

I've got my Modis here, same Troubleshooter as the Verus. It doesn't describe exactly what resistance equates to 2.5V, but it does say voltage should be about 1.5V at 185 degrees, and 2.0V at around 194 degrees.

It also states resistance should scale with temperature like this:

9500 ohms at 32 degrees
3700 ohms at 68 degrees
450 ohms at 158 degrees
250 ohms at 194 degrees

Thank you Tyler. That confirms what I was thinking-stone cold engine was reading 2.5 volts(194 degrees) while the sensor was reading 3600 ohms or about 68 degrees. The harness is making the sensor lie to the computer. Thanks for taking the time to check that for me.

Will take a month or so before I can report back with a confirmed fix

If it's resistance fall why does it's voltage go higher? It doesn't make sense for me (2.0V vs 2.5V). The higher the resistance - the higher the voltage drop on this resistor. And bad wiring should cause yet another voltage drop in the circuit so the voltage on the sensor must be even lower.

Borntoroll is right the voltage measured across the CTS should drop as the temperature increases.
My thinking is if the CTS is involved that due to wiring or internal issue the engine computer is seeing a voltage that is out of range and is going into a fallback mode a substituting a default value.

My understanding is the computer has an internal ressistor in series with the CTS and measures the voltage drop across this.

The coolant temp sensor in this case is a negative temperature coefficient thermistor. As temperature decreases the resistance will increase. As resistance increases the voltage increases-think of the garden hose analogy as you pinch the hose off(increase resistance) the the pressure(voltage) builds up behind the restriction.

I think the above numbers might have been transposed-I would have expected 1.5V=194 degrees and 2.0V=185 degrees, but either way it proved what I was thinking. In my mind the stone cold engine should have been reading over 4 volts(actual reading was 2.5) and the resistance should have been over 3,500 OHMs which it was. The cold engine was being told by the computer that it was completely warmed up(194 degrees) when in fact it is 68 degrees.