Author Topic: Cold Engine Problem  (Read 10858 times)

Offline boll213

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Cold Engine Problem
« on: July 05, 2005, 10:12:00 AM »
I just finished a 350 for my 87 4X4.  It is a driver with a few exceptions - balanced, forged pistons, everything new, small cam (Comp Cams says it works fine with ECM/TBI), fresh rebuild of TBI (I've tried a Holley replacement also - no real change).  The problem I'm having is that when it is cold, it runs rough - doesn't really have much power - I have to pump the accelerator (like a carb) to get me down the road.  After it warms up and I've shut it off and restarted - it runs fine - no problems at all.  I've changed the O2 sensor and the TPS - no improvements.  I thought about switching to a heated O2 to see if that would help, but haven't had a chance yet.  Any chance at all this could be a bad cap or wires (these are all also new - Accel, but could still be a problem)?  I've heard, what seems to be spark jumping (not much - maybe once every minute or so - doesn't really seem to effect the engine idle though) - but can't find it anywhere - tried looking at night and can't find any spark crossing over or jumping.

I'm stumped - any help at all would be appreciated.

Thanks.


Offline aprice

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Re: Cold Engine Problem
« Reply #1 on: July 06, 2005, 10:53:00 PM »
did you replace the vaccum lines? heres my thoughts, there could be a vaccum leak, and until the rubber lines heat up and expaned they are leaking, to cause it to run rough. I hope that makes sense to you, lol its been along day


Offline roundedline

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Re: Cold Engine Problem
« Reply #2 on: July 07, 2005, 09:29:00 AM »
I am just thinking aloud, (hard to diagnose without actually witnessing the problem)  Have you changed the fuel filter?  Is the IAC acting up? Did you check the previously mentioned vacuum leaks?

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project


Offline boll213

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Re: Cold Engine Problem
« Reply #3 on: July 07, 2005, 01:05:00 PM »
I believe I've found the problem - bad EGR vacuum solenoid and a VERY slow vacuum leak in the EGR valve itself.  I'm replacing them both for good measure.

Sort of on topic / off-topic question.  The TPS that I replaced isn't adjustable.  Should it be.  I checked the voltage and it seems a bit high - about .70 volts at idle - but there isn't any adjustment in the switch.  What if any problems could this cause???  Will it make it run rich?  Less fuel economy?


Offline bowtieguy

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Re: Cold Engine Problem
« Reply #4 on: July 07, 2005, 04:30:00 PM »
what about your fuel filter?????  Is it new as well?  Stumped me on my 94 TBI for a while.  Also check I believe numbers 5 and 7 plug wires, if they run beside each other they can jump spark, but that would happen at all temps not just cold.  Good luck.

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Offline boll213

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Cold Engine - Update
« Reply #5 on: July 08, 2005, 09:54:00 PM »
OK - I've installed the EGR and solenoid.  It runs a lot better, but still doesn't want to idle right when it's cold.  Now it surges.  It drops to where it sounds as if it is going to die - then picks right back up - and so on.  Same thing when you are driving at low speeds - if you step on it, it picks right up and runs smooth.  BTW the fuel filter is new.  I'm thinking IAC.  Is there any way to test it?

Thanks.


Offline boll213

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Re: Cold Engine - Update
« Reply #6 on: July 11, 2005, 09:10:00 AM »
...another update.  If I unplug the IAC before starting, the engine runs at around 2000 rpm and won't slow down.  It runs perfectly smooth, but just way too fast.  Can't adjust it down with the T20 adjusting screw either.  This makes me believe it isn't seating with 0 power - which it should - right?  If I plug it back in and re-start - it starts surging again until it warms and I restart the engine.




Offline roundedline

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Re: Cold Engine - Update
« Reply #7 on: July 11, 2005, 09:14:00 AM »

Offline boll213

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Re: Cold Engine - Update
« Reply #8 on: July 12, 2005, 07:52:00 AM »
OK - I'm giving up!  Replaced the IAC with a genuine GM (actually cheaper at the local dealer than at any of the parts stores).  Reset the computer - no change - in fact it may even be a little worse - it doesn't seem to run as well after it warms up - seems to be a little down on power.


Offline boll213

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Re: Cold Engine - Update
« Reply #9 on: July 17, 2005, 07:16:00 PM »
OK - I've been tinkering.  I downloaded WinALDL software and built an interface cable.  It works great - I'm just not sure I understand all that I know about the data 8o .  The problem is definately in the "open loop" phase of operation.  When it switches to "closed loop" at around 145 degrees, everything straightens out.  Sort of makes sense, when it actually starts reading the sensors, it adjusts and corrects the rich or lean condition (my guess at what the problem is).  So now I have the data - I just need to understand what it means.

Some additional information.  The truck was originally a 305.  I built the 350 and replaced it.  I didn't consider that there could be a problem - but maybe there is.  I've got a couple of friends who have done this on other vehicles, but not a truck.  Could this be the problem?  Do I need to change the PROM?  Are the static configuration tables that different from 305 to 350 on this model?

Thanks for taking time to help.


Offline roundedline

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Re: Cold Engine - Update
« Reply #10 on: July 18, 2005, 09:29:00 AM »
Short answer on this, the 350 IS different than the 305.  On a TBI setup, most of the time you have to change the PROM.  The 350 and the 305 from the factory use different PROMs and different injectors for the TBI motors.

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project


Offline boll213

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Cold Engine - Update
« Reply #11 on: July 18, 2005, 11:02:00 AM »
Hmmm.  Just called a local parts store to check on a PROM and they show the same part number for a 305 or 350.  I checked on a Hypertech chip and they show different part numbers for 305 and 350 - sort of expected when performance tuning.

...the saga continues


Offline 1976Scottsdale

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Re: Cold Engine - Update
« Reply #12 on: July 18, 2005, 05:07:00 PM »
Well, the aftermarket PROM form the parts store will probably be set up to run well in a 350, I have seen this a few times, many companies use the 350 as a starting point because it is one of the most common older chevrolet engines, and thus very popular and parts are often needed.  If all else fails, get the name of whoever manufactured the PROM and call them and see if it is for a 350, or head to the junkyard for a 350 PROM.


Offline boll213

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Cold Engine
« Reply #13 on: July 22, 2005, 10:31:00 AM »
...another update.

OK - I'm almost positive it is lean.  Using my monitor, I can read the IAC count at idle and it is at around 6-12 all the time (even when it's in closed loop) - which makes me believe it is lean - like a vac. leak - it's closing the IAC to attempt to richen the mixture - which it is when it is warm and in closed loop.  I've checked all lines, base gasket, EGR, brake booster, even plugged off the breather hole and placed a vac. ga. in the PCV hole to check for vac. coming from under the manifold - nothing.  

Any ideas?

Thanks in advance.


Offline roundedline

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Re: Cold Engine
« Reply #14 on: July 22, 2005, 03:39:00 PM »
The quickest way to find a vacuum leak is to spray below the carb with WD40.

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project