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Runs crappy cold...Great warmed up!?!

Kiqman

Kiqman

1/2 ton status
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Dec 14, 2004
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Rebuilt my 350 TBI recently with new heads, cam, headers. After the rebuild the truck boggs, stutters, hesitates, etc until the engine warms up. If I come to a stop sign while she's cold the motor barely keeps running. It's almost like there's water in the gas. After about 10 minutes when she's warm she runs like a champ and I'm very pleased with the rebuild.
*************PROBLEM FIXED****************************
Ok since I've had this problem people have told me to check for vaccum leaks. I only concerned myself with the vaccum lines diagramed on the engine bay sticker. I didn't realize there were others that won't react to starting fluid. Well today I notice a little black plastic vaccum line running the length of the firewall that is almost melted in two. I patched it and BAMM no more cold start hesitation AND my heater controls work for the first time ever...floor/face/windshield. Massive vaccum leak is right. Shows I need to spend more time back by the firewall. It's hard to reach the air cleaner at 5'8" let alone the firewall vaccum hose. RIght on yall!
 
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Sounds like a bad coolant temp sensor or a massive vacuum leak.
 
Wouldn't a vacuum leak continue to be a problem even after the vehicle is warmed up?
No, I didn't set the timing because that's done by the ECM on a TBI isn't it?
 
Kiqman said:
Wouldn't a vacuum leak continue to be a problem even after the vehicle is warmed up?
No, I didn't set the timing because that's done by the ECM on a TBI isn't it?
Click to expand...

Vacuum leaks make things worse when the engine is cold. Sometimes the heat of the engine can cause the gaps to close down.

Timing advance is controlled by the ECM but you still have to set the base timing. The ECM can only adjust it so much.
 
I'll have to check for vaccum leaks. I hadn't thought to since it runs so good after it's warm. As far as the base timing the shop would have done that. And again - it runs so nice once it's warm I think the timing should be fine shouldn't it? I didn't even know you could adjust the timing. Do I just rotate the distributor just like on a carburated model?
 
I had a iron intake that was cracked on the bottom side of the runner, and when cold it would suck air/oil vapors through it, and look like it was burning oil, when it got up to full temp it expanded enough to seal up, and run good. Something to keep in mind.
 
You disconnect the timing connector near near the brake booster. Single wire, tan/white IIRC. (been posted before)

Timing is a constant from the distributor. ECM adjusts timing immediately after startup (400RPM+) and when it's gone closed loop, so you'd more than likely see other issues if it was a timing problem. Timing is base timing at the distributor (say, 0*) plus whatever the ECM commands. It never hurts to check timing though, especially if someone else did the work.
 
I'm also still leaning towards a faulty/disconnected coolant temp sensor.
 
sounds like a 1 wire O2 sensor mounted too far from the heads. Switch to a 3 wire heated O2 and it will run better. My Burb ran like crap, hunted for idle, didn't rev up well, all this when cold. Switched to a heated O2 sensor and idled great, didn't stumble when revved up. Everyone is quick to think a sensor failed, sensors rarely fail when they are sitting on the fenderwell in between engine swaps...
 
A scanner is a quick "piece of mind" check in those instances that you doubt the sensors, however unlikely. :)
 
I was thinking about a heated o2 sensor. MY BMW has one that sends info to the ECM almost as soon as you crank over the engine - much better system. So how do I know which heated sensor to get for my Blazer? Do I just run the hot wire on the sensor to any 12 volt wire in my cab? Could this all just be because my ECM, when cold, thinks the stock heads are still sitting there?
 
I thought about your engine mods, but the simple fact is that it should run crappy in both situations...if the mods were so extreme as to be past the "base" open loop calibration, they would be equally off when in closed loop. You'd expect to see something like idle issues, or knock problems, or lean or rich mix, and so on when hot. Which might also tend to keep it out of closed loop.

I've got a complete different engine (essentially) than what my TPI calibration is for, yet it still runs ok open loop. A bit lean, and sometimes a bit of a swinging idle, but no different or better/worse than when hot.

If it was because the O2 sensor wasn't getting hot enough (which is what the heated ones are for, and since you have headers...) you wouldn't see the truck going closed loop, which sounds to me like where your problem is. Once everything warms up enough for the O2 to be active, the problem is either compensated for, or doesn't exist.

How does it start up when cold in the morning?

Scanner would be handy. First thing I'd be looking at is the temp sender if it doesn't start very well.
 
I don't have idle issues or knock but I do throw a 44 lean code on the freeway sometimes when I'm only at about 1/16 throttle. The 44 code goes away on it's own but sometimes I have to turn the vehicle off/on for the CEL to go off.
It starts up great in the morning regardless of how cold it is outside. I just turn the key and it fires right up. As it sits there and warms up the idle slows down just like it should. If you feather the gas it accelerates quickly and then goes back to a steady idle. Once I put it in gear though it starts acting very different. The idle goes down to about 750-800. With very light throttle it gets slowly up to speed. Any more than about 1/4 throttle and it sputters like there's water in the gas or like it's not getting fuel at all. Just for a split second mind you - and if I feather the throttle while it's doing this I can get it to stop. If the idle were any lower while cold and in gear it would stall.
Now that I'm moving the engine warms up quickly, especially with the electric fans. The dash gauge shows my temp climb to about 230 and then the thermostat opens and the temp goes to 210. Now the truck runs great. It's gone through the initial warm-up and acts great. I assume it's going into closed loop when the temp is on it's way from 230 back to 210. It's embarrasing to drive this thing for the first 10-15 minutes. If I ever try to sell it I'm going to have a hell of a time explaining why it runs like there's water in the tank until it's warm. Thanks for all the good posts again fellas.
 
I had a iron intake that was cracked on the bottom side of the runner, and when cold it would suck air/oil vapors through it, and look like it was burning oil, when it got up to full temp it expanded enough to seal up, and run good. Something to keep in mind.
Click to expand...

This has now got me thinking, my truck does the same with its L69 305. So it has a stock aluminum intake and every so often I will notice a small puddle of coolant right around the heater hose in the manifold. I double checked the hose and it doesn't have a leak. So I think this could mean that it might have a small crack in the intake.

I was already planning on putting in a new intake soon anyway.
 
I looked today and have the same little (and I mean little) drop of antifreeze next to the thermostat housing. This surprised me as I just changed the thermostat a few months ago and was sure my gasket took. I brushed away the antifreeze with my finger and it didn't come back, not even after I got to operating temp. The intake, egr, and the TBI were the only old parts I reused on the rebuild. Would a small crack near the thermostat housing effect vaccum though? Man this engine runs sweet in closed loop. I wish I could throw it in a camaro just for a weekend and see what it's really capable of. If I try it in the Blaze I'll wipe out for sure.
 
You stated a new cam, heads reworked, yet you didn't set the timing??????? True the ECM will adjust, but only after you show it where 0* is at. Start with setting the timing, then go from there. I bet timing will solve your problems.
 
The timing was set by the shop that dropped the engine in for me. If the timing were off it wouldn't run good when in closed loop, right? It runs like a top once it goes into closed loop. Has for 3 years. I'm planning on checking for vaccum leaks again this weekend as well as a possible crack somewhere in my intake manifold. Perhaps a bad EGR?
 
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