350 TBI Question - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 04-30-2014, 01:37 PM
Registered User
 
Last photo:
Join Date: Jul 2013
Posts: 21
Wiki Edits: 0

Thanks: 10
Thanked 0 Times in 0 Posts
350 TBI Question

I have a 91 GMC 1500 pick up w/ a SBC 350 tbi engine in it. The issue is the engine has a rough idle after it gets warmed up & sometimes cuts off after I come to a complete stop & put it in reverse. Here's what I've observed & repaired thus far: 1) Rear main seal leaks, 2) Changed PCV valve & the issue went away briefly but came back, 3) Smoke coming out of the oil dipstick, 4) more than normal smoke present when the truck is running & I remove the oil cap, 5) had a broken vacuum line so replaced it, 6) appears to have a back pressure issue, 7) no other vacuum leaks present.

I've done a complete tune up including plugs, wires, cap, rotor, & pcv valve. I reset the timing & advance it so that it idles at 900 rpm's when in park & neutral. The truck idles very low when in gear.....around 200 rpms.

The check engine light isn't on but does sometime come on briefly in the morning when I 1st crank the truck up. I tried to running the codes but the scanner isn't connecting to the computer (must be a wiring issue)

Anybody have any ideas????

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 04-30-2014, 03:56 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,764
Wiki Edits: 0

Thanks: 4
Thanked 426 Times in 365 Posts
Quote:
Originally Posted by mrmagic View Post
I have a 91 GMC 1500 pick up w/ a SBC 350 tbi engine in it. The issue is the engine has a rough idle after it gets warmed up & sometimes cuts off after I come to a complete stop & put it in reverse. Here's what I've observed & repaired thus far: 1) Rear main seal leaks, 2) Changed PCV valve & the issue went away briefly but came back, 3) Smoke coming out of the oil dipstick, 4) more than normal smoke present when the truck is running & I remove the oil cap, 5) had a broken vacuum line so replaced it, 6) appears to have a back pressure issue, 7) no other vacuum leaks present.

I've done a complete tune up including plugs, wires, cap, rotor, & pcv valve. I reset the timing & advance it so that it idles at 900 rpm's when in park & neutral. The truck idles very low when in gear.....around 200 rpms.

The check engine light isn't on but does sometime come on briefly in the morning when I 1st crank the truck up. I tried to running the codes but the scanner isn't connecting to the computer (must be a wiring issue)

Anybody have any ideas????
My numeral uno suspect would be the timing chain and gears are worn out causing the cam timing to fall retarded. 200 RPM idle in gear, I'm amazed it can run that slowly.

The inability to get code downloads is also a sign that the computer is failing but then again the sensors may not be setting codes, the OBD I doesn't keep track of the cam with regard to the crank so a worn timing set won't drop a code till something else is caused to fall out of spec because of the timing set.

Bogie
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 05-01-2014, 08:35 AM
Registered User
 
Last photo:
Join Date: Jul 2013
Posts: 21
Wiki Edits: 0

Thanks: 10
Thanked 0 Times in 0 Posts
So will the timing chain cause back pressure problems??? B/c there is a lot of smoke coming from the oil dipstick & from the oil cap while the truck is running.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 05-01-2014, 09:27 AM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,764
Wiki Edits: 0

Thanks: 4
Thanked 426 Times in 365 Posts
Quote:
Originally Posted by mrmagic View Post
So will the timing chain cause back pressure problems??? B/c there is a lot of smoke coming from the oil dipstick & from the oil cap while the truck is running.
Yes the late cam timing places the burn cycle regardless of ignition timing off in regard to the crankshaft this causes the exhaust to open late which holds cylinder pressure till the piston is well down the bore, this increases blow by. But I'm suggesting the timing set as one of several possible causes but being a 91 it probbaly has a lot of miles on it and these factory timing sets don't last much beyond a 100,000 miles as an average you see some die at 60-70 thousand and others go 200,000 so there is an element of crap shoot in this.

A sign of excess wear of the timing set is the need to reset the ignition timing always in the direction of advance so it catches up to the crank. A test you can do with a timing light is to disconnect the tan and black wire going into the distributor this takes the computer's advance calculation out of the equation. This is something you did when setting the timing? Then with the timing light hooked up run the engine speed up a few thousand RPM and suddenly close the throttle while observing the TDC mark on the damper. It should if the timing set is good not have deviated from TDC since the computer is out of the loop. If the timing wanders around more than a couple degrees the timing cahin and gears need replacement. However another culprit that can give the same test results is wear of the distributor gear but with the flat tappet cam in these engine that usually isn't a problem.

You can also run a vacuum test; the TBI engines should hold 17 to 21 inches of mercury at idle, probably more toward 19 to 21 since the cam is very mild. Late cam timing will produce a steady reading of 10 to 15 inches depending on how late the cam is running to the crank.

If you can confirm or eliminate that we can move on. Lots of guys the write into this forum with questions frequently do so after spending good sums of money on parts like the throttle position sensor, manifold absolute pressure sensor and other sensors only to find it's the cam timing set. So if we can rule that out for little expense we're getting ahead of the game. If that's the cause then you are putting your hard earned money into the problem rather than around it.

Bogie
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 05-01-2014, 09:54 AM
Registered User
 
Last photo:
Join Date: Jul 2013
Posts: 21
Wiki Edits: 0

Thanks: 10
Thanked 0 Times in 0 Posts
Thanks Bogie.....going to perform those test & let you know what the results are. Thanks bro
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 05-01-2014, 10:33 AM
done learning, done living
 
Last photo:
Join Date: Feb 2009
Location: Middle of the Mitten
Age: 29
Posts: 1,657
Wiki Edits: 0

Thanks: 149
Thanked 88 Times in 79 Posts
While performing the timing light test might as well check to see if the computer is actually advancing the timing. Same test bogie stated just leave the tab/black wire hooked up. You should be able to see the timing mark moving while opening the throttle
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 05-01-2014, 10:51 AM
Registered User
 
Last photo:
Join Date: Jul 2013
Posts: 21
Wiki Edits: 0

Thanks: 10
Thanked 0 Times in 0 Posts
Thanks Crussell
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
350 TBI first time rebuilding engine...question Jaykobo Engine 16 01-16-2013 02:06 PM
350 TBI to 350 TBI VORTEC SWAP IWANTMOREHP Engine 7 05-02-2012 06:40 AM
tbi 350 question americanmade Engine 11 04-16-2010 06:13 AM
87 Chevy 350 TBI rod & main Clearance question Mustangsaly Engine 3 05-20-2006 04:18 PM
need help to turn a 350 tbi into a rollerized 383 tbi Bradless Engine 2 01-03-2003 10:13 PM


All times are GMT -6. The time now is 06:19 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.