Just changed the motor in my 89 gmc k1500 over the weekend. old one was a wore out 350 k code tbi. new motor is a vortec 350 long block, pocket ported heads(906s I think) stock vortec roller cam, ebay intake and transdapt tbi adapter. Stock manifolds, oe converter and 3 inch straight pipe. Fuel pressure regulator is modded adjustable and set at 15psi for now. Fires right up and idles great. Runs great as long as the ecm thinks the motors cold. when its warmed up I have no timing advance and it falls flat and backfires out the tb when the throttle is opened up. Base timing is set at 0, advances to 5 when the connecter is hooked up at 2500 its only up to 7. tried 2 different modules with no luck. when the motors warmed up and I hook another temp sensor up and just let hang(ecm sees around 150) it runs alot better, almost good. The wierd thing is it ran good before I changed the motor. Double checked to make sure all the grounds were clean ad hooked up and nothing was pinched between the bellhousing and the trans. Getting really frustrated. Ecm is the 7747. Any input would be great.
Nick

 

I would check to make sure that you Are at tdc with your distributor and check your timing again. With stock programing you should have about 18-20% of timing at idle with the est wire connected. without it you should read 0%. You may also have a bad coolant temp sender or a bad esc module. Not to sure though. You do have the est wire hooked up right?? Hope that this helps a little.

 

Timing is definatley right, Ive changed it several times trying to eget it to run. Tried a different ecm half hour ago with no difference. Last night I had a volt meter hooked on the wire that sends the timing signal to the ign module. at idle it was around 1.6 volts. 2500 it was close to 3.0. with the temp sender seing 195 about half throttle in gear voltage was around 2.0. If I leaned into it much farther the voltage would drop to 1.6 or so and it would backfire out the tb and just fall flat in general. If I hooked the other temp sender up so the ecm saw 150 instead of 195 about half throttle would still be 2.0 volts but if I opend it up past there the voltage would be at 2.5 or more and the rear tires started spining. Why am I losing my timing advance when the ecm sees a warm engine? I went out when I got home and started it up, let it run for a couple of minuits and went for a drive. when the ecm sees cold it has all kinds of power. When it sees 180 or more it falls flat. Getting real irritated. and yes that is with the timing connector hooked up. I checked it last night and all 4 wires between the ecm and the dist are in good shape. Would really like to get this figured out.
Nick

 

Nick are you burning your own chips or did someone burn one for you?

If so, its pretty easy to make a mistake in setting the various tables and options that control the timing. For example, the "Initial Spark Advance" constant is subtracted, not added to the total timing.

Here's a thread with a pretty good overview of ECM timing:

https://www.thirdgen.org/forums/diy-...calculate.html

Paul T.

 

Still running the stock chip.

 

You're looking in the wrong direction for your problem. The timing not advancing is most likely just the stock timing tables. Unlike a mechanical distributor the stock timing tables are 3 dimensional in that they are based on RPM and vaccume. So untill you get the motor under load you won't see much of a timing change.

The real problem here is that your ECM still thinks there is a stock motor parked under the TB. The speed density style FI system that TBI uses works by looking up how much fuel to inject by going to a table that is RPM and MAP based just like the timing tables. The problem you run into is that when the engine needs more or less fuel at that particular RPM and MAP reading the ECM has no way of knowing. Your vortec motor needs substantially more fuel and a timing curve thats drastically different from stock. What you are seeing while it's warming up is that the ECM dumps in more fuel, once it leans out the mix when the engine is warm you are too lean and this is causing your drivability problems. As a crutch you can get a stiffer spring for your FPR and try to get some more FP. The real fix is to learn to burn your own chips and fix it right. Check out the "Free Tune" thread for a primer.

 

it sounds like it runs good in open loop but goes bad in closed loop. is your o2 hooked up and ok? what about your knock sensor? did you use your old one. you have a roller cam now. which may require a different knock sensor.

 

I had figured before I started this project that the vortec would take more fuel and had bumped the fuel pressure up to 15 to compensate, But this thing is more than just a little lean. With the ecm hooked up to the temp sender in the intake and base timing set at 0 it will start fine and idle fine. But when I try to drive it it will barely move without backfiring. Would it be that lean or do I have something else I need to find. I can go out right now when the ecm sees less than 100 degrees and it flat hauls a$$. If Im down to needing a chip how much does the basic equipment for burning a chip cost? For right now I just went to radio shack and bought an adjustable resistor to wire in with the temp sensor. The more resistance the ecm sees from the temp sensor the better this thing runs. For right now thats going to be my crutch. I dropped the motor in it last thursday and have been fighting it for over a week now. I need my truck drivable. As for the knock sensor I used the one from the vortec because I thought I had read on here that the roller ones take a different one. The o2 sensor is a new bosch and appears to be working. And running the fuel pressure up to 18 pounds didnt seem to help. According to the gm tech one from work when the power starts falling off the o2 sensor goes rich and stays there. Thats why I was sure it was in the ignitiion.
Nick

 

It's more than likely all in the tune. Anything more than full exhaust will benefit greatly from a custom tune.

Your new motor has much much better heads and a better camshaft. I'm surprised it'll even idle.

Check out the "free tune" sticky at the top of the forum and see if that will get you started.

 

Figured out today that 90% of my problem is lack of timing. Went out this morning and checked timing. Ecm seeing about 160, base timing set at 15 with connector unhooked. reconnected and idling timing starts at 30 but after about 30 seconds slowly drops back down to 15(base timing). If I run it steady up to 3000 timing only advances about 10 degrees, which is the same as when the connector is unhooked.Decided to experiment and bump the base tinming up to 20. I then hooked on to my car trailer with my stock car on it(about 5000# or so) and drove it 180 miles round trip to one of the specials that was running this afternoon. The first 25 miles or so there are some pretty nasty hills that are usually a chore. Everysingle one of them I was never much over 65 at the bottom and never less than 55 at the top. After that there was about 60 miles of 4 lane with mild hills and a 15+mph headwind. Had the cruise set at 70 in 3rd and never down shifted, never dropped below 60. On the way home the tailwind had dropped to about 10, same thing cruise set at 70, this time in overdrive, 2000 rpm. Bareley working it. Only kicked into third 4 times in 60 miles. ac on the whole way. Temp guage never moved. After that Im convinced something is screwed up with the timing, I just dont know what. That was today, yesterday I hauled the same car and trailer 15 miles over to wayne with the timing at 15 and it was dead over 2000. Ive set timing on numerous tbi trucks over the years at anywhere from 0 to 10 degrees and they ran fine. There should be no reason that this motor needs 20 degrees extra timing over the crap tbi swirlport heads. Every dyno test that I have read details on for race motors have the vortec timing only around 32 where the standard heads are 36-38, That being due to the vortec having a more efficient chamber. Why does the timing drop back down to base? And what kind of voltage should I be seeing from the spark control module? Checked it yesterday and its around 9.8 volts and doesent seem to change. Just trying to figure this out.
I fully understand that this thing will not run perfect with completley stock parts and that some tuning is required. However something is not right here. I can understand lying to the ecm about the engine temp to put it a little higher on the fuel curve to make up for the added airflow. But this there is something not right with the timing. Just about need to tap into the ecm on a similar year good running truck and see what kind of voltage the ecm is seeing from the different sensors and what voltage its sending out to the distributor module for the timing advance.
Nick

 

Here's a better idea, get a cheap aldl cable or build your own. Then you can datalog your truck and see what is really going on. No more guessing no more wondering.....see it in black and white. Stock ECM's won't give you your timing, but will allow you to see what the knock sensor is doing (trust me you'll want to see what it's doing) If you get into chip tunign I'll show you how to get your actuall timing put into the aldl stream.

Seriously take my word for it, your tune is just off that far. If you can give me the numbers off the chip in your ECM I can look it up and post up exactly how much timing you should be seeing under idle conditions and free revving in nuetral. Just to get an idea of what I was talking about earlier, have some one put the truck in gear and load it against the converter, you should see completely different timing numbers than when in nuetral.

Guys have swapped to the LT1 cam and haven't been able to get out of the driveway. Your motor swap is about 2 times the difference than the LT1 cam.

 

base timing of 15 degrees? geez. way too much. Maybe 2 degrees with the wire DISCONNECTED and that's it. Your engine should be idling with about 16-17 degrees in idle with the wire CONNECTED.
Definitely gotta go the DIY tuning route. How do the spark plugs look?

BMonteSS- FWIW my original setup was '7747, $42, ARJU (california emissions). This is for a 350 TBI (K), 700r4, A/C, Calif. EMissions. His is probably very close to mine.