Pls Help! 350 TBI Swap - High Idle - Knock Under Load

[nick9004]

Hi Guys, I recently got my TBI swap running. It runs and drives great. But I have 2 issues and I don't know if they are related nor how to fix them even with all of the searching I have done on this amazing forum. First some background info.

81 GMC Jimmy, Fully rebuilt 350, TBI installed with stock freshly rebuilt TBI heads, Flat top pistons, Mild RV Cam, Edlebrock Performer TBI Intake, All sensors hooked up except VSS. I’ve been running it on premium fuel which here is about 91 Octane. ECU is a 7747. The donor vehicle is somewhat unknown. I bought the TBI parts off of a friend who had it running on a 377 de-stroker without changing the EPROM. I am using the same EPROM. My SES light is working but is not on while I’m driving. it only comes on for a few seconds when I turn the key. I am assuming this is a test to show that the light is working.

Problem #1: Ever since the first start-up it has had a very high idle. I don't have a tach but when I shift into gear it’s a lot more shock to the drive train than there should be. Also the idle does not change after the engine heats up.

Problem #2: I have a knock under load. Only under load and regardless of if the engine is hot or not. If anything, it gets more noticeable as the engine gets warmer. During conservative driving it is nonexistent. However, conservative driving is no fun especially when you want to see what your new setup is capable of. It is mainly noticeable if I'm rolling up a slight grade and want to accelerate. The RPM begins to climb and it knocks but if I keep it in 1st gear and get the RPMs up high it disappears completely. Then drop to 2nd still on an incline and give it more gas it knocks again until the RPMs get up. Also I can make it do it if I am in drive and stand on the brake while holding the RPM up. I understand the concept of detonation/pinging but am not sure of how it sounds. The distributor is currently set at 0* and it seems to be slightly more noticeable than when I have it set at 4*. I would like to test the knock sensor but when I follow my Haynes book directions for testing the knock sensor both the sensor I’m using and a brand new sensor fail. The way I’m testing it is testing the resistance between the block or and engine ground and the pin on the sensor. I’m using a digital multi-meter and its telling me there is no connection between the pin of the sensor and the block.

Does anyone have any other ways of checking the knock sensor or any other suggestions for me? As for the high idle I can’t hear or see any vacuum leaks and the throttle body gasket is good and tight the idle screw still has the factory plug over it and I don’t want to play with that before I’m sure there is no underlying problem that it would mask. I may ahve access to a laptop this weekend so I will try and do some WinALDL Datalogging.

Sorry for the lengthy post!

Thanks a lot in advance. This forum is so helpful!

 

[dave w]

I'm thinking maybe the compression ratio might be high? TBI heads are about 65cc (castings 193/191). Not knowing the part number of the flat top pistons, not knowing the piston compression height of the flat top pistons, I could be correct / incorrect?

A couple years ago I disassembled a Knocking SBC 350 with TBI heads and flat top pistons. The compression ratio was almost 9.8:1 Anyway the engine knock caused a spun rod bearing.

I rebuilt the engine using dish pistons, re-sized connecting rods and a reground crankshaft. The engine runs fine now.

Maybe not everyone will agree with me, but I feel 9:1 compression ratio is the safe limit for 91 octane fuel.

I would use a scan tool like AutoXray http://www.autoxray.com/product_detail.php?pid=94 to check for knock counts. I know, the AutoXray is an expensive tool

dave w

 

[nick9004]

Hey, thanks for the reply! I thought about that. But when picking out pistons at the machine shop that was doing all the machine work for me we figured it to be 9:1 which like you said is a resonable limit. That is my reason for running the higher octane feul. I'm thinking it is more to do with some tables in the EPROM to do with timing vs RPM. Does that make sense? I'm shopping for a laptop and an ALDL cable instead of a scanner. Seems like it might be more useful than a scanner?

 

[dave w]

I'm thinking it is more to do with some tables in the EPROM to do with timing vs RPM. Does that make sense? I'm shopping for a laptop and an ALDL cable instead of a scanner. Seems like it might be more useful than a scanner?

I attached a screen shot of the PROM tables, and expanded the spark table. Not sure which PROM broadcast code (BBC) your ECM has, so the tables I attached could be a little different.

I got my LapTop used ... about $200. The ALDL is about $60 available from http://aldlcable.com/ Not sure if WinALDL will show Knock Counts? I have not used WinALDL recently. I usually end up using both the Scanner and WinALDL to figure things out.

 

[nick9004]

Dave, Thanks for the screen. Ive got both a laptop and aldl on its way. Unfortunatley I won't see either for about 2 weeks. After seeing your screen and reading lots of posts here and on thirdgen.org its all making a little more sense now. Once I've got some info I'll post it.

 

[Keitha]

Have you gone trought the process of setting the IAC, Throttle Position Sensor, and your Idle speed? The three are interrelated and may help your 1st issue.

 

[nick9004]

Hey Keitha, I tried that this past weekend. Seems to have helped slightly but still sounds high. Once I get the laptop I'll be able to see what my idle really is. But im sure its still high.

 

[Thunder]

First of all there should not be any ground between the connector and engine block on a knock sensor. If the circut is closed between the connector and ground the KS is bad. Knock sensors are a piezo crystal sensor. When they detect engine knock they generate a small voltage that the ESC module picks up and tells the ECM that the engine is knocking. The ECM will back off timing till the engine quits knocking.
If the knocking wont stop after timing is retarded to pre programed specs in the ECM A code 43 will be set if timing is retarded 20* and still knocking.
If there is no code 43 you either have a bad ground to the ESC module ( retangular black box next to the passenger side of the TB) Bad knock sensor, short to in the wire that goes to the EST module in the dist. or possibly a bad ESC module.. These usually dont go bad and a wireing problem or bad ground is usually the problem.
Check out the ESC circut then post up.

 

[MIKESMONZA]

A back up test for the Knock Sensor

For those of us with out a Scan Tool At hand

I know A realy cheap way,,
to Visualy Check if the Knock sensor is working!

#1 -A timing Light Aimed at Balancer Timing Line,
,on a runing engine,

#2-Then,, Just take a Hammer or round rod and start tapping/Striking
the Heads, Block or accesory brackets,,

>>>> If it is working it will retard and move the balancer Line away from Advance!

>>>Also if you ever make the mistake,,
of Installing a Noisy Timing Gear Set Up;
On any Gm Vehicle with Knock Sensors
for Example;

>Like a "Pete Jackson" Set up or simular Device!
It will retard as soon as it start's
Mike