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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.
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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
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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
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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?
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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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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.
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...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.
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It sounds like you need to replace the IAC.
Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project
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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.
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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
. 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.
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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
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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
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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.
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...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.
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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
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Thought I would take a minute to post an update on my problem. It wasn't a vac. leak. It was/is a fuel problem. It is lean, but it is because it isn't getting enough fuel. I put a new Fram fuel filter on prior to installing the new engine. After exhausting all of the possible vac. problems, I switched my focus to fuel. I bought a new Wix filter and installed it. About 80% of my problem immediately went away. Still a little off on power when it is cold, but runs. I cut the Fram filter apart and it was completely clean, just a bad filter. I'm not a Fram fan!
The balance of my problem, I believe, is fuel pressure. I'm only getting about 8 lbs. at idle, I'd like to get that up to about 12 -13 lbs. (read somewhere that was the target pressure). Any ideas other than changing to an adjustable regulator. I'm wondering if a stiffer spring or shimming the existing regulator would be options???
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Are you getting 8 before or after the regulator? How much are you getting from the tank. It should be between 13 and 16 from the tank pump. If you are only getting 8, that would be a little low. GM recommends between 9-13.
I am not saying this is the problem, but it is one place to look.
Here is more info on GM TBI: www.cfm-tech.com/gm_tbi_f...essure.htm
This is a link that should assist in making the regulator adjustable.
www.thirdgen.org/newdesig...afpr.shtml
www.diy-efi.org/gmecm/faq/tbi_fp.html
Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project
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Thanks - I took the regulator apart this weekend and raised to pressure to about 12 (that's all I could get out or it without shimming or replacing the spring). Runs a lot better, power is good, just a little surge at idle when cold, but runs just fine under load.
Thanks for all your help!
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my son and i are doing the same basic thing. go to tbichips.com for lots of info. the things different on the 350 from the 305 are: injectors,prom,electronic spark contr,and knock sensor. do the heated o2 sensor will stop the surging. good luck.
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Do you have an application or a part number for a heated or 3 wire O2? Also, how will this help - as I understand the way the computer works, it will not read the O2 until the engine reaches a pre-determined temperature - read by the temp sensor in the intake and will continue to use the "static fuel matrix" until then. So, heating the O2 wouldn't really make any difference if the computer isn't in "closed loop" and using that information to adjust mixture, etc. would it? Perhaps changing the computer is also necessary in order to make this fix work?