Truck "stumbles" when cold

[1985K10]

I just got my 87 GMC on the road...350 w/tbi, runs perfect, except when i first fire it up, and pull out of my driveway and take off, at low rpms(light throttle) it kinda stumbles or bogs i guess...till u push the pedal harder then it takes off good. After about 2 minutes of driving its fine.

For some reason i suspect the EGR valve...anyone have any other ideas?

[killthewabbit]

If it had a carburetor I would say choke. Not sure how that (warm up) works on TBI, though.

[grinch4om]

it's not the EGR. check the engine coolant temp sensor, if it tells the computer its warmer than it actually is, it'll run lean until it warms up.

[Rebeltilldeath3]

Check vacum lines as well. I was having similar problems, except it would last no matter what the operating temperature was at.

[burnitdown612]

sadly to say but sheck the little cpas on the throttle body, iv had them come like the tiniest bit loose and when i would first start my 87 the low end rumbly vibration would make it do that, or also check your temp sensor, the ECM sometimes gets confused and tells it to run leaner than it should

[1985K10]

How do I "check" the temp sensor? Or should i just change it?

Thanks guys!

[7387chevys]

Originally Posted by killthewabbit

If it had a carburetor I would say choke. Not sure how that (warm up) works on TBI, though.

Thats what I would have said until I readon and saw that it is a fulie.........Im dumb when it comes to computerized cars so heres a ttt for you
Shane

[grinch4om]

www.autoshop101.com/forms/h32.pdf -

I did a google search for , "engine coolant temperature sensors" and this one seemed to explain how the sensor works better than I can in this setting

[rfmaster]

'85K10

A typical CTS is nothing by thermistor - a resistor that changes it's value with temperature. An ECM sends a calibrated reference voltage (+5V) and looks for a voltage drop across CTS. A Delco 12146312 CTS has the following resistance over temp (C ) performance:
Temp Resistance
-40 102,122
-20 28,939
-10 16,321
0 9,516
10 5,728
20 3,555
25 2,830
30 2,268
50 992
70 475
90 244
100 180.3
120 102.2
140 61.1

So if it's a cool morning (say around 10C) then resistance will vary from 5,728 Ohms down to about 285 Ohms when fully warm. You can check CTS with a DVM by disconnecting connector from the sensor and measuring resistance when cold. You can repeat the same measurement after engine run for a while. In both cases ignition must be off or otherwise ECM will throw error codes.


//RF

[magnethead]

On similar topic- my truck is running on what appears to be 7 or even 6 cylinders when it's cold at first- It vibrates the truck like an old diesel (and sounds like one too almost because of how the exhaust note is affected). But when it's warmed up it's fine. No plugs are arcing, i'm thinking i need to do a compression check maybe? 454 cid motor with quadrajet on top. My other idea is maybe my choke is out of whack (i'm about ready to rip the thing off there or just wire-tie it straight vertical all the time) because it keeps playing games with me, but would that cause it to run as I describe?

[1985K10]

Well, cold, I measured the resistance and it came out to 5 deg C, or 41 F, which was the exact temp here this morning. After I warmed it up to full temp, I checked again and it came out to 195 deg F....So it seems the CTS is functioning correctly...Im pretty sure theres no vac leaks...Is there something else to check out?

[7387chevys]

Originally Posted by magnethead

On similar topic- my truck is running on what appears to be 7 or even 6 cylinders when it's cold at first- It vibrates the truck like an old diesel (and sounds like one too almost because of how the exhaust note is affected). But when it's warmed up it's fine. No plugs are arcing, i'm thinking i need to do a compression check maybe? 454 cid motor with quadrajet on top. My other idea is maybe my choke is out of whack (i'm about ready to rip the thing off there or just wire-tie it straight vertical all the time) because it keeps playing games with me, but would that cause it to run as I describe?

Wiretied the choke straight vertical on a q-jet until I got an new carb .....electronic carb+non electronic truck = headaches
Shane

[magnethead]

Originally Posted by 7387chevys

Wiretied the choke straight vertical on a q-jet until I got an new carb .....electronic carb+non electronic truck = headaches
Shane

That's what you did? How much harder was it to start on cold mornings?

[rfmaster]

‘85K10

In TBI-220 system idle speed is controlled with Idle Air Control (IAC) Valve. This is the four pin connector stepper motor valve which basically provides additional air during idle, since throttle plates are closed. It stops adding additional by-pass air once engine speed exceeds a predetermined RPM (varies depending on the vehicle type), if Throttle Position Sensor (TPS) reads above 5%, Manifold Absolute Pressure sensor reading, CTS, ITS and other factors which I do not recall at this time. A full description is rather lengthy.
Back to IAC – it acts as a controlled vacuum leak. IAC will try keep idle close to pre-programmed speed (typically 650) by moving IAC control piston in & out. Any dirt, carbon build up will screw things up at idle. IAC is located at 11:00 position when viewed from front. Clean the IAC pental (head) with isopropyl alcohol – do not use any harsh cleaners! Clean out internal passages with a conventional carburetor cleaner.

Do you see any codes set by ECM?

I hope this helps

//RF

[7387chevys]

Originally Posted by magnethead

That's what you did? How much harder was it to start on cold mornings?

Didnt stay like that long just until we got a new carb.............it kept it from running (as)bad .......just a temporary fix.
Shane

[1985K10]

I didnt check for codes...and the service engine light never came on.

I will try to clean the IAC, if it affects idle speed, I should also mention no matter what temp, the truck seems to idle a little high...

Thanks again!

[1985K10]

I cleaned the IAC...didnt seem to fix anything...

[HuggerCST]

One possibility- My book shows an 87 to have 2 coolant sensors- One for the computer(in the head), and one for the gauge(in the front of the intake).

[rfmaster]

85K10

We tried obvious things with sensors. Looks like both coolant temperature sensor (CTS) and inlet air ( IAS) sensors are functioning – ECM is not showing any codes??? In some TBI systems IAS is not used and instead manifold temperature sensor (MTS), very similar to IAS is used .

Check for vacuum leaks. Spray carb cleaner around vacuum houses see if you have cracked vacuum hose.

EGR valve. EGR valves are known to have some carbon build up at the port. EGR that is not fully closed will cause engine to idle rougher or stall all together.

Off idle stumble may also be caused by a dirty injector. Take a timing light – connect it as you would normally do for timing and point at the injector spray. Both injectors must produce even amount of spray.

Ignition – check wires for damage and plugs for fouling.

//RF

[1985K10]

I'll check the CTS in the head tomorrow...I didnt know there was one...As for dirty fuel injectors, I wouldnt think the engine getting warmed up would affect them that much...

Can I remove and clean the EGR valve, or would replacement be a better option?

I'll check the vac lines again too.

[rfmaster]

'85K10

You can check the functionality of EGR valve very easy. Disconenct and plug vac hose at EGR. If you have a hand held vacuum pump attach it to EGR vauum port. Give it couple of pumps (say around 10 PSI of vacuum for 30 seconds, I consider it OK.) EGR should open and engine idle will get worst, but ECM should be able to compensate for lean-out. This test may set engine check light -but you can manually clear it.
From there cleanning EGR passage is next step - unbolt EGR clean out carbon build up in the cavity/seat - also always replace EGR valve gasket. If EGR is blown you will not see vacuum being hed by pump or the drop in idle. If EGR shot - replace.

CTS is typically located by thermostat housing, to the left. It uses black/white (System Ground) and yellow (CTS Sensor) wires in the harness.

There are other sensors to check (TPS for wear), but check obvious staf first.

//RF

[lurch115]

Originally Posted by magnethead

That's what you did? How much harder was it to start on cold mornings?

its more than likely the choke is closed to far....you should have a "choke pull off" on the passager side of the carb. check and make sure that it isn't bad... and if its not it should also have an adjuster on it...what that choke pull off does is pull the choke open more once the vehicle starts and has vacuum...basically prevents the choke from sticking shut... if the choke isn't opening enough it will cause the engine to lope and shake because the cylinders ar loading up with too much gas and not enough air

[magnethead]

Originally Posted by lurch115

its more than likely the choke is closed to far....you should have a "choke pull off" on the passager side of the carb. check and make sure that it isn't bad... and if its not it should also have an adjuster on it...what that choke pull off does is pull the choke open more once the vehicle starts and has vacuum...basically prevents the choke from sticking shut... if the choke isn't opening enough it will cause the engine to lope and shake because the cylinders ar loading up with too much gas and not enough air

sounds about right..Do i just turn the adjustment screw right 2 turns so it latches it in more of an "open" position?