1987 Chevrolet 350 tbi high idle

[AuroraGirl]

I believe it’s a three wire. Two going into a harness and one ground that is a few inches from the sensor. I don’t know the part number off head but I’ll look it up. How do I go about tested the one on the head? I have a multimeter, I think I know what the sensor is it’s between the first and third cylinder exhaust. No I don’t recall the prom chip. I have no idea about anything to do with the computer on this truck. Should I go back and re read what you’d said and try it? Also the idle was around 1200 but today it’s been at 1600 and kicks up to 2500 when I press clutch or put it in neutral. Driving sucked this morning.

yes the prom chip you should take the computer and lightly tap it when discharged for static, and also remove and reseat the chip in the ECM. carefully. ensure its good. They can cause some weird stuff.

 

[AuroraGirl]

yes the prom chip you should take the computer and lightly tap it when discharged for static, and also remove and reseat the chip in the ECM. carefully. ensure its good. They can cause some weird stuff.

Ill go find the FSM stuff to help, but just know if the system is operating in a failsafe for a part of the ECM it sometimes will set a default action that will "work hopefully" Like increase idle speed to account for super cold temps and ensure the battery charges some. But that fail safe stuff isnt just a simple "code" if its happening. Your truck, in my opinion, is thinking the temp is -40 or a combo with the IAC not being adjusted right if you messed with it at all. also the IAC may be worn.

 

[AuroraGirl]

Im gonna dump a bunch of TSB that may be relevant also

 

[AuroraGirl]

Couple more.

Operation​

The coolant sensor is a thermistor (a resistor which changes value based on temperature) mounted on the engine coolant stream. As the temperature of the engine coolant changes, the resistance of the coolant sensor changes. Low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F), while high temperature causes low resistance (70 ohms at 130°C/266°F).

The ECM supplies a 5 volt signal to the coolant sensor and measures the voltage that returns. By measuring the voltage change, the ECM determines the engine coolant temperature. This information is used to control fuel management, IAC, spark timing, EGR, canister purge and other engine operating conditions.

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this is also the proper inset for the fan. i think yours is in too far.

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there is your sensor. the blue dots that black wire, is shared between TPS and the ECT. The shared wire should be a 5 voltreference and that is going to stay as close to 5v as the computer can do, which is pretty darn close under most conditions. that should not ever pull low or go high and shouldnt be more than .03 volts usually one way or the other minus a quick dip but recovery if like alternator gets taxed possibly. That wire should be checked for integrity and any disruptions and make sure key on its working. the other wire to the PCM would be the one which will read based on the sensors temp. The ground you identified goes to the engine somewhere. Can you show me that ground?

 

[AuroraGirl]

merry christmas lol i gave you a lot to read and mull over. any questions ill try to keep an eye on the forum

 

[iamtherealJayy]

I’ll start reading those when I get home from work, I ordered an aldl usb cable to hook to a laptop and I’ll start downloading programs to try and figure something out. I believe the ground is on the alternator bracket I’ll try and send a picture showing it

 

[iamtherealJayy]

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I’m not sure how to circle it but I’ll use the photo from yestetday, the wire that bends and goes down beside the thermostat housing it is bolted down beside the thermostat housing.

 

[AuroraGirl]

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i dont have a TBI esp a 87 but this also doesnt seem to be your issue but the fact it was called out in the TSB makes me think its either not supposed to be there or it was a troublesome place and perhaps it was moved or just is best monitored closely

 

[GTP51]

this can be a deep dark hole, I remember some stuff about it , first thing to Remplacements is the IAC from memory I got an AC Delco one and it did help.
turned out the issue was throttle shafts were were worn out in the the TBI. As soo as we changed that away she went . One other thing on the TBI ‘s is vac leaks, I replaced every and I mean every hose and each little bit helped, but it was the TBI that was the main culprit, I know it’s not what you want to hear but , I spraye carb cleaner and all sorts of stuff around to try and bring up the usual tell tales.
‘good luck.

 

[Rusty Nail]

Dude. Your truck is broken.
Dont drive it that hot...what is wrong with the radiator or SOMETHING, this thread is scary omg does it got any OIL in it?
Bad water pump or radiator or - cracked head - or

Ad infinitum

You can watch the temp rise and fall when acceleration water pump forces coolant over the sensor maybe.
The

Was the fan spinning?

The coolant should be topped off through the overflow with the radiator cap installed.
Hell , IT could be the problem. The cap.
Plugged radiator.
Cracked head.
Ya know a Loose Nut Behind the Wheel is a common issue for these old trucks...

Take the shroud off.

Ad Infinitum.

Best to call a mechanic I guess. Hope its not too late.
I was interested in the fan being in the shroud whoever said that gots a good eye. That really is an issue even I can see.

I'm at cracked head.
Sorry bout your luck but mebbe now you can get super cool VORTEX heads.

And some new engine bearings....maybe some new pistons...i'll never build another 5.7 inch rod motor again...6 inch rods or bust!

 

[iamtherealJayy]

Yes lol, it’s got oil 5 fresh quarts of it and not the cheapo stuff either. The entire cooling system is new besides the heater core hoses themselves. I replaced radiator both hoses heater core and now like the 5th thermostat I’ve put in it. Yes the fan spins, and the shroud could be sat wrong, the truck was totaled if you recall, the bottom of the shroud is cracked it was used from old parts after wreck. I don’t personally know if it could be a cracked head due to it runs fine and operates fine with a lower thermostat but it doesn’t like 180 or 195. But hell more temps is more pressure right? Maybe the 160 never let it build enough pressure to cause an issue? I’ve got no idea. This all started because I was tired of the truck screaming all the time sitting at 2000 rpms and now I’m about to be in the hole several hundred if not thousand dollars… also @AuroraGirl can you please explain your letters, example TSB, I know things like ECM but I don’t know all the terms sorry.

 

[AuroraGirl]

Yes lol, it’s got oil 5 fresh quarts of it and not the cheapo stuff either. The entire cooling system is new besides the heater core hoses themselves. I replaced radiator both hoses heater core and now like the 5th thermostat I’ve put in it. Yes the fan spins, and the shroud could be sat wrong, the truck was totaled if you recall, the bottom of the shroud is cracked it was used from old parts after wreck. I don’t personally know if it could be a cracked head due to it runs fine and operates fine with a lower thermostat but it doesn’t like 180 or 195. But hell more temps is more pressure right? Maybe the 160 never let it build enough pressure to cause an issue? I’ve got no idea. This all started because I was tired of the truck screaming all the time sitting at 2000 rpms and now I’m about to be in the hole several hundred if not thousand dollars… also @AuroraGirl can you please explain your letters, example TSB, I know things like ECM but I don’t know all the terms sorry.

TSB is technical service bulletin its something from a manufacture usually to dealer technicians and it covers a lot but often its "common issues" or "fixes" they want to convey as to speed up time on their ends, like if they need to use new hose clamps(one of the things I sent) then it explains how to get them why and what to do.

ECM engine control module
PROM programmable read only memory (MEMCAL/PROM chip that is in the ECM, the computer. memcal is memory calibration, another word gm uses for them)
ECT engine coolant temperature - sensor
EGR exhaust gas recirculation
TPS throttle position sensor
PCV positive crankcase ventilation
IAC idle air control

did you read things? no need to assume a cracked head we havent even gotten out of the base diagnostics

I think we are looking at an issue that isnt so complex, rusty said something I did too, check the cap replace it. get it filled. get the fan set in the shroud proper (needs to be closer to the engine) Do you have a big fan, if you could mock up a shed fan in front of the radiator while idling to move air i would tell you to do that to see if that fan would make a difference. but your idle is too high and i think it could be ECT (coolant sensor) which also can be affected by air pockets esp next to the thermostat.

 

[Babs]

I had a high idle problem with my 88 Suburban 5.7 tbi and it ended up being the tbi base gasket was worn and damaged. I changed it and idle went back to normal operation.

 

[gmbellew]

from the pics, the fan doesn't look that far out of place to me. plus running down the hwy at 45mph shouldnt even require much fan for cooling. but it is still worth looking at closer, I suppose.

the water pump, as suggested is a good thing to check. and things that drive the water pump. is the pulley tight, is the serpentine loose, etc? although if you get good heat out of the heater, coolant is likely circulating.

I am guessing the heating issue is separate from the idle issue.

on the idle issue, I still lean toward IAC issues or an unfound vac leak.

for the over heating, I'd check the actual block temp (thermometer or aldl reader) and check to make sure t-stat is opening at the right temp. if those check out, I'd hit up the service manual for your year and start looking at the coolant system diagnosis in the Reference Library and start making the checks.

 

[iamtherealJayy]

How would I go about adjusting the fan closer to engine? The fan spins with engine running and changes with rpm. The cap is new as well replaced it when I replaced radiator. Could it be the spring in radiator? I believe there were different sizes? Maybe? I can’t remember. I just got recommended by oreillys. It was around 30° this morning on my way to work, the heat was nice but I left it turned off hoping to keep temps down since it was sitting between 210-260, so not sure how a fan moving air would help any better than it already being below freezing outside and moving 45-55mph. @AuroraGirl where is the ecm and prom stuff located? I can look into it but I’m not sure where it is. Also is it easy to mess something up if it start messing with it? Or is it mostly straight forward? I assume disconnect battery it messing with that so you don’t short anything? I haven’t gotten to reading all the files you sent above but I’m going to try to when I get back home.