Cold start vs Hot start, is this normal?

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jcardona1

jcardona1

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I think I've read all the relevant threads on here about the problems with hot starting. Doesn't seem like it's related to my issue.

'96 LC. First off, I haven't had any instances where the truck won't start. My trucks starts. On a cold start it will crank once and fire up instantly. But when the engine is hot after the truck has been running a while, it seems like it has trouble starting. The engine will crank several times then finally fire up. When it fires up, it seems "sluggish". It's hard to explain, but it definitely doesn't sound as 'confident' as it does on a cold start.

Any ideas what could be causing this? Where do I start?
 
Check your timing. Sometimes when a rig gets hot and the timing is off it can struggle to start and run smooth.
 
On a hot start, once the truck is running does it run ok? Any different than if started from a cold start?

Is the problem isolated to starting then once running everything is normal?
 
Curious if you ever solved this, I'm have the same issue and haven't made much progress. It's not a problem per se, but it's also not right.

New items to rule out, FP regulator and FP relay, cap/rotor/plugs/wires, battery, correct ignition timing, charcoal canister and lines, 02 sensors. I'm wondering of it's maybe the ECT sensor, but that seems like a long shot.
 
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Curiouss also having the same exact issue... Slow start when hot.. and yes I mean hot after running up to temp in Phoenix were it's been 115 F last few days..
 
For me, it's not that hot. I just mean the engine is warmed up normally. It runs fine once it cranks for a bit and stumble starts. I'm not sure when the TB was cleaned last, but that's on my short list too.
 
I experience the same problem. When cold it hits right away. Hot start it needs to turn over a few times before it starts. Once it starts it stumbles for a second then idles and runs fine. Weird. My ignition parts are fairly new, TB cleaned, Timing checked. I just accept it as normal.
 
polloelastico said:
But my OCD tendencies want to reject this as normal!
Click to expand...
I hear you buddy... I can't let it go.. IT IS NOT NORMAL!! It should start slower when cold and faster once warmed up.. It defies logic.. I have a new starter, 1 month old battery + fusible link.. All ground connections cleaned and secure.. also cleaned the Throttle body at the same time.. I will do the fuel filter soon.. But there is no reason for it to slow start on a hot start.. my OCD won't let me forget it.. :rolleyes: :doh:
 
If it were me, I would probably take the idle air control valve off (if I remember correctly, you'll have to remove throttle body for proper tool access) and clean any moving parts. The brass screws that hold it on are soft, so be careful not to strip them.
 
Ok, I'll be sure to check/clean the IAC valve, but I'm skeptical about that being the issue.

I'm also going to verify the ECT sensor and EGR components, some of which will be much easier when the Throttle Body is off. Might be a few weeks before I can get to this though.
 
Generally the IAC is not as much of an issue on these engines it seems. I've owned 3 different 3.4L trucks and I had to clean the IAC very often due to lagging hot/warm starts. Worth a shot though.
 
bajaphile said:
Generally the IAC is not as much of an issue on these engines it seems. I've owned 3 different 3.4L trucks and I had to clean the IAC very often due to lagging hot/warm starts. Worth a shot though.
Click to expand...
I'm confused by this answer- IAC cleaning (for you) worked or did not work as a PM item for warm/hot start issues?
Thanks!
 
Well, I was hoping to have solved this but I've made zero progress, even though over the last year I've done quite a bit of PM and replacement of worn or questionable items. I am still getting the stubble and rough hot start, but only after it sits for a bit (maybe 15+ minutes or so), if it's less then it fires right up. Again, cold starts are fantastic.

I have no idea what is left to check. I have done:

  1. Clean TB
  2. Sent injectors to RC Engineering (they were bad, but no reports of leaking)
  3. Replace all vacuum lines and valves under the intake while it was off, in fact I've replaced every rubber hose and vacuum line on the entire truck at this point.
  4. Valve cover gasket and tubes (relates to vacuum)
  5. Inspect and rewrap harness
  6. All the emissions and EGR related things
  7. Replace IAC and gasket on throttle body
  8. O2 sensors are new (whole exhaust system is actually)
  9. Fuel filter
  10. Fuel pressure regulator
  11. Fuel pressure relay
  12. Fuel pressure resistor
  13. EFI related fuses (and fuseable link)
  14. Fuel pump and filter sock
  15. AIr intake hose
I mean, what else could control this? TPS? I'm wondering if it really is a leaky injector that wasn't caught during cleaning, but that seems weird.

Once it's running it's fine, but it gives the truck a "crappy" quality that bothers me and it doesn't seem that good for the motor maybe.
 
I’m with all of you on this. My rig had always been this way. It is ‘crappy’ and a little frustrating . . .but also my normal with the ‘96 80 series I’ve had since 2015.
Things I’ve changed that haven’t made a difference:
Fuel pressure regulator
Fuel injectors
Fuel filter
Fuel pump and FP relay
Plugs
*timing is all good*
Crank position sensor
Rebuilt evap canister
🤷‍♂️
It’ll be interesting to see what finally solves this issue. At times it just fires right up and others I’ll crank for a while . . .

My pile leaked literally every fluid when I first picked it up(don’t get me started) and I was sure that the leaky injector was to blame. Nope.
 
Well, during the lockdown this past fall I did a head gasket and valve job, replaced the engine harness with a new Toyota part as mine had damage near the EGR valve, sent the injectors to RC to have rebuilt, new fuel filter and still experience this issue. I dont think its quite as bad though. I have a theory as to what might be causing it. When the engine is cold, the ECM is in open loop mode and is operating on a set of preprogrammed parameters. For cold starting the fuel tables are probably set to run rich. As the engine temp warms the ECM sees the ECT signal and goes into closed loop at a set temperature and at that point starts accepting inputs from various sensors like oxygen sensors etc. At normal engine operating temp the ECM leans the air fuel mixture. I almost think when the stumble happens its a lean condition. If your engine has a lot of carbon on top of the pistons like mine had I think that might make the lean condition worse. I didnt replace my ECT sensor when I had the head off but I wonder if it could be that? I also know that the TPS is very critical to engine function and needs to be set accurately. I have noticed that although my hot stumble hasnt disappeared completely, it is not as prevalent as it was. Any thoughts on this?
 
Interesting. I did replace the ECT sensor, so I can rule that out.

I haven't touched the TPS because I'm a little unsure of how to apply vacuum according to the FSM and to verify it is set properly. Using my ODB2 scanner the TPS value usually shows up around 11% at idle. I did find a thread here that indicated that was likely acceptable. That said, it does seem like a path to go down and verify it's set properly. I've found most things the PO did were not, so if there is a small chance it was touched, I'll assume it's not set right.
 
No idea if this would help, but if someone has an old/spare temp sensor, try unplugging the normal (hot) engine coolant sensor and plug the connector into the spare sensor (tricking the ECU to thinking the engine is cold). Would be interesting to see if it has any impact....
 
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