Hard hot start - temp sending unit, how to diagnose?

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MatSLO

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First, a quick description of my problem: sometimes (not always but often) my Tahoe would have difficulty starting the engine - it cranks fine but would take several seconds before actually firing up and when it does, it starts firing kinda sluggish. Once it starts, it idles, runs and drives nice and smooth.
(when the engine is cold, it always starts immediately, just touch the ignition key and she purrs)

This summer, I had the starter replaced - with the old (original) starter, I occasionally had trouble starting in all conditions. With the new starter, it was much better, but the occasional hot start problem persists.
Note: the new starter is POWER SELECT 6449N, the cheapest one I found on Rockauto.com:
https://www.rockauto.com/en/moreinfo.php?pk=6680808&cc=1061367&jsn=843
I still have the old starter and only later did I notice that it has the heat shield on - either the mechanic forgot to transfer the heat shield or the new starter didn't accept it.

Now the question(s):

- could there still be an issue with the starter?

- after researching the hard hot start issue (also on this forum and FB group), temperature sensor was mentioned - now the question is:
- which one of the sensors is it, the one on top of engine, or the one on the driver's side of the engine near the plugs?
- how to test the sensor - will a OBDII scanner (I heve a cheap one) show anything funny with the sensor that may cause starting problems?
 

bugdewde

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Let me start by saying that I'm not a mechanic, and really don't know what I'm talking about.

Cranking fine, just not firing off .... Typically, wouldn't be a starter issue, unless it was just cranking too slow to get the whole thing rolling. Starter is an electric motor, either it works or it doesn't (typically).

My '95 454 does the exact same thing. After a good heat soak, it cranks fine but doesn't start for a few seconds.... Starts immediately/instantly when cold.

I'm tending to think the heat is doing something to impede the firing process. Reduced ignition spark (coil) or fuel pressure (pump).

Just a thought :
Next time it's hot, and you expect it to not fire quickly.... try cycling the ignition switch to run (not start) to let the fuel pump build pressure (3-4 seconds?). Switch it off, then back to run to pressurize some more for 3-4 seconds .... Maybe 3 or 4 times .... To build fuel pressure before starting.
Not sure if it will actually increase fuel pressure... Depends if your fuel pressure regulator is doing it's job.
 

454cid

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There's a good chance the heat shield did not fit the new starter. That's what I ran into with mine. I have no problems with it missing, although it doesn't get very hot in my area.

I would suspect it's a fuel issue. Either low pressure from the pump, or possibly the spider assembly. I have no experience trouble shooting the spider assembly, as my truck doesn't have one. You can check the fuel pressure easily enough, However. The engine has a test port.
 

PlayingWithTBI

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after researching the hard hot start issue (also on this forum and FB group), temperature sensor was mentioned - now the question is:
- which one of the sensors is it, the one on top of engine, or the one on the driver's side of the engine near the plugs
I'm not sure on the Vortec engine but on the earlier TBIs the CTS they're referring to is the one next to the T-Stat housing. But I agree with @454cid it sounds like low fuel pressure (min 60PSI), almost like you're getting vapor lock. As @bugdewde suggested cycle the key a couple times see if that helps any.
 

east302

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- after researching the hard hot start issue (also on this forum and FB group), temperature sensor was mentioned - now the question is:
- which one of the sensors is it, the one on top of engine, or the one on the driver's side of the engine near the plugs?
- how to test the sensor - will a OBDII scanner (I heve a cheap one) show anything funny with the sensor that may cause starting problems?

The coolant temp sensor (CTS) is the one by the thermostat, the one on the driver side block is for the dash gauge.

A scanner could be used to see what temperature is being reported, sometimes it will show some oddball value. A workaround is usually to hold the gas to the floor while cranking, this is the “clear flood” mode and temporarily stops the injectors.

There’s another method for testing the sensor (think it is looking at resistance vs ambient temperature) but I’m drawing a blank right now.
 

MatSLO

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Thanks for the input so far.

The fuel pump is relatively new, it was replaced 2-3 years ago after the old one died. Otherwise, when the engine starts, it runs smooth, always enough power, no hesitation or cuting off.

I'll first look into possible malfunction of the temperature sensor (which I also had replaced as a precaution, 2-3 years ago when the fuel pump was replaced; on that occasion, intake manifold gasket was also replaced along with a full tune up). I'll check the coolant temperature with the OBDII scanner and see if it's showing anything strange.

It's also possible that there is a problem with the fuel pressure regulator - but if I start digging this deep in the engine's guts (or pay the mechanic to do it), I might as well pull the trigger and replace the old injection spider with the new upgraded version. But I'd like to explore cheaper options first.
 

MatSLO

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So today I hooked up my $25 OBD scanner and monitored the coolant temp.
I did this in the afternoon when driving home from work. The truck had been parked for 7-8 hours from the morning drive.

Upon starting (it fired up almost nomal but not 100%), the coolant temperature read 70F (outside temperatures were somewhere in the mid 50s thoughout the day).
During the 4-mile drive, the coolant temperature stabilzed at just under 190F.
Before arriving home, I stopped at a nearby store to buy some stuff (temp was 189F at the moment of engine shutoff), the truck was sitting for 5 minutes or less. Right before starting the engine, the temperature on the scanner still read 189. The engine started slightly sluggish (cranked fine but "came to life" a bit hesitantly), after which the coolant temp dropped a few degrees before settling at 189F again.

Tell me if I'm wrong but the coolant temperature readings seem to be prety much fine.
I guess it's either the fuel pressure regulator or the spider... any thoughts?
 
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The coolant temp sensor (CTS) is the one by the thermostat, the one on the driver side block is for the dash gauge.

A scanner could be used to see what temperature is being reported, sometimes it will show some oddball value. A workaround is usually to hold the gas to the floor while cranking, this is the “clear flood” mode and temporarily stops the injectors.

There’s another method for testing the sensor (think it is looking at resistance vs ambient temperature) but I’m drawing a blank right now.
Read by checking resistance across the two prongs on the sensor.

Try a hot start with your foot to the floor (the flood clearing mode described earlier) could have an injector leaking down when the manifold gets heat soaked after shut down. My geuss is it's fuel related.
 

east302

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Tell me if I'm wrong but the coolant temperature readings seem to be prety much fine.
I guess it's either the fuel pressure regulator or the spider... any thoughts?

CTS readings look fine. Try the “gas to the floor” next time, but a fuel pressure test would probably be in order.
 
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