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Username Post: 1987 chevy truck stalls under load with low idle        (Topic#265964)
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-10-11 07:10 PM - Post#2112746    

Hello,
I have an 1987 Chevy truck TBI which has an annoying stall. It happens usually when I turn the wheel to the max and put a load on the engine (while maneuvering at idle speeds), or when coming to a sudden stop. It seems after the engine Idles down (after start up) it wants to idle a little on the ruff side and lower than normal. almost like the timing bypass is unplugged, low idle kind of ruff. (but its not)It does have new tune up parts (plugs, cap, rotor, fuel filter) I did time it to 0 degrees like the information said ( with the wire disconnected, sticker on the header panel). no change. I did put a new control module in and the crazy thing is after the truck would warm up it would die. If I disconnected the timing bypass wire it would stay running (a little ruff, but running) so I reinstalled the old one and it would stay running but same result. I did try more than one control module and I even had one of the new ones tested. (it tested good). I also tried a temp sensor with no change. I did pull the codes before I started changing parts and I had a 42, that's why I went to the control module. Today when I was driving the ck eng light came on. I checked the codes and I had a 32 (EGR) I did ck the EGR for free movement, which was good. I know the code is related to the EGR control not the EGR valve itself, and I don't thing it is related to my stalling problem but wanted to mention it. Any help will be greatly appreciated
Thanks

 
gchemist 
Senior Chevytalk Moderator
Posts: 21870
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
07-10-11 08:03 PM - Post#2112770    
    In response to zippymailman

Welcome to the site. Bookmark this page: http://www.cfm-tech.com/GM_tbi_tuning_tips.htm

How many miles does the TBI have on it? Engine codes for TBI FI are sometimes hard to pinpoint to something specific. You can add a few degrees of timing from 0. Have you checked fuel pressure? Compression on each cylinder? Remove and clean the air idle valve. Use a good TBI cleaner. Run a can of Seafoam through the tank. How old is the fuel filter? Is the EGR hose in good shape? Does the EGR valve stick? Clean it and air blow it. How good are the grounds? Check the battery to frame, engine to frame, and other grounds.
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles


 
Vaughn 
"12th Year" Gold Supporting Member
Posts: 15398

Loc: Colorado Springs, CO
Reg: 08-08-04
07-11-11 10:08 AM - Post#2112966    
    In response to gchemist

A low rough idle is a CLASSIC sign of a stuck-open EGR valve.

The fact that you might be able to get the pintle to move does NOT mean that the EGR valve is OK.

EGR is shut off when the engine is cold - and is activated when the motor is warm via the coolant temperature switch (CTS). EGR valve should ONLY OPEN at constant mid RPM with the engine under load (like when traveling down the road in 4th at 2000 rpm).

If there is a problem with the control or with the vacuum hose routing, the EGR can be activated at idle - which should never happen, because it CAUSES the rough idle and stalling.

You can disconnect the EGR valve as a test condition, if the engine does not idle rough or die WHEN WARM with the EGR valve disconnected, your control unit or the vacuum hoses going to the EGR valve are at fault.

If your truck still idles rough and dies when warm with the EGR control disconnected, you need to replace the EGR valve.

While the timing specifications for your truck indicate that it should be set for 0 degrees with the timing reference signal disconnected, your motor will run MUCH BETTER with 4 degrees or 8 degrees BTDC with the timing reference unplugged, and it will get better gas mileage.

 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-11-11 07:37 PM - Post#2113155    
    In response to Vaughn

Ok, I will give that a try and see what I come up with on the EGR issue. What do you make of the Code 42 and the issue with the control module? I thought it was very strange that it would run better and than within a few minutes die, it did it with every new control module (2). and when I replace it with the old one it would stay running. The idle is low and more stumbley than rough.

 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-11-11 07:45 PM - Post#2113158    
    In response to gchemist

The truck only has 61,000 miles on it. It looks as if there is a good spray pattern when the truck is running, although I haven't checked the fuel pressure. Nor have I done a compression check. It seems to run fine anywhere above idle. The fuel filter is fairly new (3000-5000 miles), grounds all look good. I will check the (IAC valve, I think that's what it is called.) Any thoughts on the control module issue?
Thanks

 
gchemist 
Senior Chevytalk Moderator
Posts: 21870
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
07-11-11 09:07 PM - Post#2113194    
    In response to zippymailman

Code 42 may be bad due to wiring. Check the connections carefully. Basically open circuit or short or component failure. Have the first module tested at the parts store. Check if the distributor is receiving full voltage. Check the alternator and battery connections. Check for full power.
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles


 
Vaughn 
"12th Year" Gold Supporting Member
Posts: 15398

Loc: Colorado Springs, CO
Reg: 08-08-04
07-12-11 05:21 AM - Post#2113253    
    In response to gchemist

You should deal with the trouble codes one at a time - because often the first trouble code is the problem, and second/third/etc are codes that are thrown because of the first code.

EGR does affect timing. Recirculating exhaust gases through the intake causes the combustion gases to be not-as-hot inside the cylinder (it also cools the block temperature) - so the TBI will advance timing on the motor when the EGR is active. This might be why you have a code 42 also, because that is the ignition control.

 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-13-11 07:17 AM - Post#2113637    
    In response to Vaughn

OK, The code 42 is what I had to begin with, the code 32 came a couple of days later while driving above 45mph and the the ck eng light stayed on. when I started it the next time and it was off. I am assuming that the EGR control is not working properly because that is when the EGR valve should open (when I had the light come on, higher RPM's) Going to do some investigating today and hopefully find something.
Thanks

 
someotherguy 
Moderator
Posts: 25580
someotherguy
Age: 44
Loc: Texas
Reg: 08-01-03
07-13-11 07:20 AM - Post#2113638    
    In response to zippymailman

Vaughn is spot-on about suspecting the EGR. Here's some troubleshooting info: http://www.chevytalk.org/fusionbb/showtopic.php?ti...

Richard
94 C2500LD / 94 C1500 / 06 300C SRT8
Check out my truck shop projects


 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-13-11 07:27 PM - Post#2113885    
    In response to someotherguy

OK, I ended up replacing the EGR valve, the idle did smooth out and pick up nicely. But, (there's always a but!) I took it on the expressway and the the Ck Eng light came on again. I am assuming that it is the EGR control module. And as I am pulling in the drive I was turning the wheel to the limit and I felt it stumble. So I let it idle as I was continuing down the drive and when I was at the point of almost stopping I was turning the wheel back and forth and it stalled. I started it up again and put it in gear (reverse) quickly and it stalled. How disappointing! any suggestions

 
Vaughn 
"12th Year" Gold Supporting Member
Posts: 15398

Loc: Colorado Springs, CO
Reg: 08-08-04
07-14-11 10:15 AM - Post#2114093    
    In response to zippymailman

Get the code off of the computer first.

Did you advance the timing like I suggested above?

 
Bad Bowtie 
Valued Contributor
Posts: 4536
Bad Bowtie
Age: 42
Loc: TX
Reg: 07-04-03
07-14-11 12:50 PM - Post#2114136    
    In response to zippymailman

  • zippymailman Said:
OK, I ended up replacing the EGR valve, the idle did smooth out and pick up nicely. But, (there's always a but!) I took it on the expressway and the the Ck Eng light came on again. I am assuming that it is the EGR control module. And as I am pulling in the drive I was turning the wheel to the limit and I felt it stumble. So I let it idle as I was continuing down the drive and when I was at the point of almost stopping I was turning the wheel back and forth and it stalled. I started it up again and put it in gear (reverse) quickly and it stalled. How disappointing! any suggestions



Do you even know if the exhaust passages are cleaned out?

You can install a new GM EGR valve, but it does no good if carbon has clogged up the holes in the intake manifold.
GMC. We are Professional Grade.

http://www.cardomain.com/id/2004VHO


 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-14-11 06:41 PM - Post#2114256    
    In response to Vaughn

I ended up changing the EGR selenoid and that fixed the CK eng light. still have the stalling when turning the wheel to the limits, also stumbling in the morning when I start it. acts like its wants to die and then rev's up to keep itself running. I was going to change the timimg but I was side tracked. somebody suggested that I check the Idle air control valve to see if it was gummed up. I pulled it off and it had a light film of what looked to be cigarett tar. I clean it up and re-installed it. The engine stalled as soon as I got it started. I purchased a new one re-installed it and the idle was very high. The instructions indicated that the truck should be drove at 40mph for about 4 miles for the computer to reset the IAC valve??? I did that and the Idle was still extremly high. re-installed the old one after I turned the plunger in a couple of turns and the truck stayed runinng. unplugged it as the truck was running and no change in the idle. I plugged the new one in as it was running and no movement on the plunger. Where does the plunger get its input from. I was told that the plunger would be moving rapidly back and forth. I really need some help with this. Thanks

 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-15-11 12:53 PM - Post#2114472    
    In response to zippymailman

I put the old IAC valve in and it starts but with a low idle. the Idle will climb as the truck warms up???
Any suggestons

 
Vaughn 
"12th Year" Gold Supporting Member
Posts: 15398

Loc: Colorado Springs, CO
Reg: 08-08-04
07-15-11 06:30 PM - Post#2114553    
    In response to zippymailman

The reason that I wanted you to adjust the timing is because it will have an affect on the idle speed. You should do this FIRST. Do not forget to disconnect the timing reference signal wire before changing the timing, otherwise the ECM will back out any timing changes you make.

The IAC control needs to be reset on whichever IAC you use. This is why your idle is so far off.

You shouldn't have messed with the set screw on the old unit. Small inputs on this screw can change the idle speed dramatically. Your old unit would have been correctly adjusted if you had only cleaned it and reinstalled it.

You can reset the valve using this procedure:

http://www.ehow.com/how_7959677_reset-bas eline-idl...

Be extremely careful to use a new gasket with whatever IAC you use, even tiny vacuum leaks will dramatically change rpm.

If you want an in-depth explanation of how the IAC works:

http://diy-efi.org/gmecm/papers/747iac.txt

 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-15-11 07:35 PM - Post#2114572    
    In response to Vaughn

initially I pulled the IAC valve out cleaned it and re-installed it. I started the the truck and it would stall immediately. I pulled the IAC out again and noticed that if I turned the plunger it would lengthen or retract the shaft. I thought that as I was turning the entire valve out that the plunger may have been stuck and was turning out as well, so I turned it in a a few turns re-installed it and the idle was very high. I didnt mess the base idle on the TBI at all. I realized that I may have screwed this up and went to purchase a new one, installed it and the idle was extremely high. The instructions indicated to drive the vehicle for a while at 40mph and shut the engine off and re-start and the IAC would reset to correct position. Didn't happen. I then removed it turned the plunger out on the old IAC valve a little installed it and the truck runs but still has a high idle and it gets higher as the the truck runs more. I did plug the new one in to see if i could see any movement while the truck was running and I saw no movement. I believe that the IAC was working before because I would have a warm up idle and then it would decrease, or if I unplugged the temp sensor the idle would jump up. I don't think I can re-adjust the timing while at such a high idle. I did read in the information you supplied that the IAC is responsible for bumping up the idle when the engine requires it (A/C on, low voltage ect.) and then the computers takes over to keep this idle as a base until condition changes. the initial ondition I had was when turning the steering wheel to limits the idle would drop thus causing "low voltage" but no idle compensation and that's why I think it would stall. Is the ECM the only component that controls the IAC? Also the other link of information was to set the base idle on the TBI correct? (if so I didnt adjust it, it still has the cap on it) Is there any way to check to see if I have the required voltage changes or voltage at all in IAC pigtail?
Thanks for taking so much time, I really appreciate it.

 
Vaughn 
"12th Year" Gold Supporting Member
Posts: 15398

Loc: Colorado Springs, CO
Reg: 08-08-04
07-16-11 02:26 PM - Post#2114763    
    In response to zippymailman

There are no timing advance mechanisms left in the distributor. They have been removed and are controlled by the ECM now. Disconnecting the timing reference signal unhooks the ECM from the distributor, so no advance is fed to the motor.

The ECM is the only thing controlling the IAC valve.

Turning the key off and on resets the IAC valve. It can also be reset by jumping the A and B terminals on the ALDL connector.

IF your IAC valve is installed correctly - and IF the IAC valve has been reset a few times - then you either need to adjust the pintle with the set screw OR you have a vacuum leak that is increasing the idle considerably (probably a brake booster hose/PCV hose/MAP sensor hose has become disconnected).

 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-17-11 07:52 AM - Post#2114938    
    In response to Vaughn

Just a bit of info:
The idle increases as the truck warms up. I just dont think the IAC is getting any input I figured the IAC valve is controlled by the ECM but the ECM gets info from where so the IAC can make corrections? (MAP sensor?)I plan to replace the TBI gasket today and take a good look at the IAC valve when I have it off. I was not aware of a set screw to adjust the pintle. I will look for that as well. The high idle problem started when I removed and replaced the IAC valve so I am assuming the it is related to that but I will check all the vacuum sources too.
Thanks again for the continued effort and supply of information.


 
Vaughn 
"12th Year" Gold Supporting Member
Posts: 15398

Loc: Colorado Springs, CO
Reg: 08-08-04
07-17-11 04:23 PM - Post#2115061    
    In response to zippymailman

The ECM gets the information to adjust the idle from:
The current RPM
The Coolant Temperature Switch (CTS) - whether in open/closed loop
The O2 sensor
Whether the AC compressor is on
The power steering pump
and the MAP sensor.

It reflects these values against a table of values in the EPROM, and if a number of values matches the table, it increases the RPM.

 
zippymailman 
Forum Newbie
Posts: 10

Reg: 07-10-11
07-19-11 02:53 PM - Post#2115812    
    In response to Vaughn

well, I have no vacuum leaks and I reset the IAC valve, timed it at 4 degrees BTDC and it does idle better but still stalls. Mostly right when I start it now, after I drive it down the road it seems to "perk up" a bit. How does the ECM get input from the power steering pump? I didn't know it had any electrical hook up? can you shed some light on this. Thanks again

 
gchemist 
Senior Chevytalk Moderator
Posts: 21870
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
07-19-11 06:16 PM - Post#2115907    
    In response to zippymailman

GM started using sensors on power steering to regulate idle/pressure on later models. I'm not sure when GM started to use the feature. I doubt your 87 has the input. If it does, it's the first time I have heard of it.
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles


 
Vaughn 
"12th Year" Gold Supporting Member
Posts: 15398

Loc: Colorado Springs, CO
Reg: 08-08-04
07-22-11 11:12 AM - Post#2116818    
    In response to gchemist

How did you check for vacuum leaks?

What is your vacuum gauge reading?

Did you try advancing the motor more?

Timing makes more difference to a motor than you realize. GM publishes much tamer timing recommendations than it should, basically because of variations in production line tolerances (one motor might knock at 4 degrees, while another may not knock until 14 degrees). There are long an complicated reasons for this, but for now just advance the timing.

ECMs have the ability to read power steering pressure - if there is a demand for more pressure at idle (like when parallel parking) - the ECM reads the pressure drop and then kicks the motor idle higher in order to make sure the motor doesn't stall because of power steering pump demands. I don't know if you particular model has this sensor, it may not have been available yet.

 
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