89 gmc knocking under load 350 TBI

[hatchetman]

I have a 89 gmc that idles fine but knocks when accelerating.Ive covered most the basics but I need some suggestions. Ive checked for loose or missing bolts in the flywheel/torque converter(that sounds to be the origin) and they are present and tight. Dont know if this is computer or sensor related, mecahanical or what, but it gets really bad when climbing an incline.The 'knock' can be more defined as metal hitting metal from the bottom of the truck/transmission area- not like a rod knock (besides that ,its not there at idle or when reved in park or neutral). Any suggestions would help! Im not a fulltime mechanic-just on weekends! Thanks.

[ChevyTech]

It could be a cracked flex plate (the part many people refer to as a flywheel). They crack in the area where they bolt to the crankshaft. Sometimes it can be very hard to see the cracks without removing the flex plate.

Does it sound like pinging?

Did you check the EGR operation at all?

Did you check for ECM trouble codes?

Does the check engine light come on while driving?

[hatchetman]

I checked the EGR and it was working. Trouble codes(42&44) went out when I replaced the manifold oxigen sensor and coolant temp sensor. Im gonna replace the knock sensor next,but havent had the trouble light back on at all. I just went to the gas station and put top grade gas in it and it improved slightly. Thats what it seems like to me is an engine running on bad gas or is out of time(the timing WAS off by 2 deg. which I set to 0) That cracked flywheel makes me wonder though. The truck was sitting for a few years but I have reciepts from the original owner for a rebuilt tranny,which,shifts a little hard. I just bought the truck about three weeks ago for a firewood hauler so my brother and me have been trying to undo a few years of neglect. On a curious note I was following proceedures to test for knock sensor opperation and got no change in timing when I rapped the engine(I still claim to be no mechanic!!) Thats pretty much where I'm at now and I appriciate the input!

[ChevyTech]

A code 42 is set when the ECM loses communication with the distributor.

Code 42 can be set by a bad distributor module, a bad ECM or wiring problems between the ECM and distributor.

A code 42 will get set when the set timing connector is unplugged to set the timing. Make sure you clear the codes after setting the timing.

A bad distributor module is a common cause for this code.

Code 44 is a lean condition.

A bad knock sensor should set a code 43.

On a curious note I was following proceedures to test for knock sensor opperation and got no change in timing when I rapped the engine(I still claim to be no mechanic!!)

When the system detects a problem and sets a code 42, it runs in a backup mode where electronic spark timing control is not used, and the system runs with less timing advance.

Some vehicles have to be in drive for the electronic spark timing to control timing. If you don’t have someone to help hold the brake, be careful and put the truck on jack stands so it does not get away from you. Don’t trust the parking brake to hold it.

If I were working on the truck, I would clear the codes and see if they reset. If the code 42 returns that is the code I would fix first.

[hatchetman]

that makes sense because the last time the light came on is when I unhooked the timing wire and set the timing and tryed the knock sensor thing.I had someone hold thier foot on the brake in drive for the knock sensor but not to set the timing.Is that correct? So now I should reset the codes since timing and keep my fingers cossed. I really apprieciate your help!

[ChevyTech]

Yes, you have someone hold the brake, with the transmission in gear, on some vehicles, for testing the electronic spark control system.

All you need to do to set the timing is unplug the set timing connector. Then after the timing is set, reconnect the set timing connector and clear the trouble code.

You can clear the ECM codes by removing the ECM fuses for at least 10 seconds.

Some people disconnect the battery to clear the trouble codes, but then the radio and other systems loose their memory too.

Always make sure the key is OFF when removing power from the ECM. Preferably the key is in your pocket, to be sure.

[hatchetman]

Well I cleared the codes and went for a ride. No engine lights at all but that out of time/lack of power is still there.After cleaning up my dads leaves I took him for a ride and he agreed that it sounded like it was out of time by the the sound of the clatter.So if the timing is on the mark and its clattering and lacking power, what then-valve job,collapsed lifter or could there still be something electronic causing it? Thanks again.

[ChevyTech]

The timing can be set right on the mark, but be wrong because the outer ring on the front pulley can slide so that is no longer accurate.

What is the clatter you refer to? Does the engine “ping” – “detonate”?

If the engine does ping, the knock sensor will detect the pinging and cause the ignition timing to get retarded, by the electronic spark controls built into the system.

The knocking noise you mentioned at the top of the thread will cause the timing to retard.

Engine rattles and knocks from lifters, cam, rod, crank and piston noises all have the potential for causing false knock signals to be produced.

The Knock sensor contains a piezoelectric crystal which causes a voltage to be produced. Loose components on or in the engine can cause the knock sensor to create a voltage and the system may interpret this as pinging. The system retards the timing because it determines it is pinging.

If you hear something knocking, don’t expect it to run well.

[Boog]

I'd try advancing the timing by 2* and drive it. If it shows improvement advance it another 2*. These early TBI engines can take a few additional * of timing. Also some guys use a product called Seafoam to clean carbon buildup from the combustion chamber. I have used it a couple of times and can tell a difference but moreso in my 305 equipped trucks than the 350's.

[hatchetman]

I probably shouldnt have said 'knock' so much as 'rattle' from the valvetrain like you would hear from bad timing. The knocking is more when you'd put your foot in it,but again, due to timing ,more like a harder rattle. Also the lack of power to climb hills is sickening! I did use seafoam right after I changed sparkplugs and cap/rotor and it looked as if it did its job judging by all the carbon that came out of the pipe (left a trail!).I live in an E-check county in Ohio and just got back-it passed(YEE-HAW!) I suppose I could try to advance the timing as was suggested but it just seems like something from the time it goes from park to drive is misinterpreted by the computer. I say that because other than driving down the road the engine sounds perfectly healthy and starts well. I am told these trucks were problematic in the fuel delivery like improper pressure but supposedly all thats been replaced. I guess at the end of the day I should rephrase the original question to "What would cause predetonation under load,but otherwise run fine??" Again thanks to both of you for your help.

[ChevyTech]

I guess at the end of the day I should rephrase the original question to "What would cause predetonation under load,but otherwise run fine??"

EGR not working
Lean mixture
Incorrect timing
Heavy carbon deposits in combustion chambers

Again thanks to both of you for your help.

You’re welcome

How did you test the EGR?

[BigBlocksRule]

If it was babied before, it could sure be carbon in the chambers.
Get a spray bottle full of water and give it a spray into the throttle body every few seconds at high idle. That's a SPRAY, not a STREAM. If it's carbon that's causing your problem, you'll see it come out the exhaust pipe.

[hatchetman]

Well truthfully, I dont know a whole lot about the EGR system. A friend of mine who was helping me with it unplugged the line and said it seemed okay. After reading a little about it I think I'll rethink some things. I messed with the timing a couple of times today but no improvement no matter what degree I put it in. Problem is I got about an hour after work before its pitch black out and my garage is full of my 70 chevy! As for the life of the truck and weather it was babied or not- for the first ten years of its life it hauled cars back from Texas to Ohio for restoration. The 2nd ten years it was driven a little and then pretty much sat there in neglect as an extra truck. As I said before I sea foamed it and alot of soot came out so I know its full of carbon. No doubt the thing had a rough life but hey- its a GM, they thrive on that stuff! But its the sitting around that kills them. Im definatly gonna look into the EGR a little harder like you told me about 4 posts ago! BTW, Ive been working with some general contractors from Minnesota off and on this year. They're good people. Thanks again.

[ChevyTech]

Being you saw soot out the tail pipe when using Sea Foam, it is possible that the EGR, or the EGR passages, are plugged with carbon.

Try this:
If you lift the EGR diaphragm with your fingers, with the engine idling, (being careful not to burn yourself or get your fingers caught in the EGR valve) you should hear a definite difference in the way it runs and the engine may stall. If there is no difference you should look for plugged passages in the intake manifold and EGR valve.

Let me know what happens.

BTW, Ive been working with some general contractors from Minnesota off and on this year. They're good people.

Many general contractors here are scraping by, not working, or have left the business. I see many Craig’slist ads that look like contractors selling all of their tools & equipment.

[Ol'Rover]

lol i had the same problem with my 98 GMC 305 vortec... once it started doing it it took 6 days untill it stopped running lol.... i will soon find out what caused it to happen

[hatchetman]

I got a chance to corner the company mechanic today, who was able to keep two of these same trucks in service 'til about 300,000 plus miles before they sold them two years ago running! He said "first of all, change the fuel filter!" Then said basically what you just did about the EGR. So I think before I get stuffed tomorrow afternoon I may mess with the EGR in the morning unless I can find an open part store to get a filter.
That was the impression I had with the contractors when they came all the way to Ohio to build a Verizon terminal(the place where they handle the transmissions not where they sell cellphones)! Work is picking back up here,hopefully it does there as well. Happy thanksgiving!

[ChevyTech]

A fuel filter is definitely worth a try. A lean condition sure can come from a restricted filter.

I would test the fuel pressure also being is set a code 44 (lean code).

[hatchetman]

I pushed the diaphram on the EGR and it bogged the engine so I am to assume its working? went to loosen the fuel filter and discovered a few stripped nuts on the lines. What a mess!! I hit it with some PB blaster and figure I'll let it bake for a day until I get a filter tomorrow. I bet Ill be replacing the rear fuel line soon! Gotta love living in Ohio where they salt the roads heavily. Welp cheers and heres rust in yer eye!

[ChevyTech]

Here in Minnesota we know all about rust.

I pushed the diaphram on the EGR and it bogged the engine so I am to assume its working?

That is just the first test.
Now we know the EGR passages are not plugged is carbon.

If you have a hand vacuum pump, apply vacuum to the EGR valve with the engine off and see if the diaphragm lifts and holds open. Then start the engine and see if the valve closes.

[66truckin]

Are your bellhousing bolts all tight? Its not a th400 trans is it? I have seen bellhousings on them crack and break off causing a knock.

[hatchetman]

Well,I think I found the problem. I was taking the fuel filter off and looked back towards the tank and noticed the lines looked a little wet. I went ahead and got a filter and put it on but suspected that when it started I would see dripping. Low and behold it did! So sucking air through the fuel line may cause a lean condition? Probably so! I think I might get my favorite mechanic after this one, he needs to eat too! Im lookin at it like fixing the exhaust; not that difficult but you can have it done for about the same price to do it yourself!
If this dosen't work I'll let you know.

[ChevyTech]

It won’t suck air in a leaky fuel line because the fuel pump is in the gas tank and the entire supply line is pressurized.

The return line has no pressure in it, and if it leaks it won’t cause a lean condition either.

[hatchetman]

It dawned on me Ive never closed this post-sorry! What was this problem? Turned out to be a blown headgasket on one side and a failing one on the other side.Hope this helps someone out there!

[ChevyTech]

I do wonder sometimes about how things work out for people.

Thanks for the update.

There was a post a week or two ago about intake EGR passages full of carbon. The guy was changing one head gasket and found the passages full of carbon.

The head gasket failed between the two center cylinders.

His original complaint was pinging.

He is now replacing the head gasket on the other side of the engine for leakage between the center cylinders.

[Blue_71]

wow, if this wasnt resolved i was going to tell you to check the headgaskets my 89 with 254K miles is doing that.. i tried all new ignition system, playing with the timing, different known-good computers.. finally one day it started missing... i pulled off the #4 plug wire... no change... pulled off the #6 plug wire... no change.. tested the compression, and it was EXTREMELY low on 4 and 6.. this is a big problem with the TBI engines i have found