Hi Guys,
New to the forum, and i'm looking for some advice/help.
I have a 1988 GMC S15, that I swapped a 1997 Tahoe 5.7L Vortec & 4l60E into. I kept the factory engine harness and ECM. The ECM was tuned by blackbear to get some stuff taken off that was no longer in use.
Over the past few months I've been trying to track down some gremlins that have been popping up.
I should note, that I left the engine completely stock... For now.
After i tore down the original engine from the Tahoe, I decided to get a long block from O'reilly since they price matched to someplace i found online for $1100 bucks!

Fast forward to today, and the truck is running and driving. However, on startup there is a noise coming from the engine bay, it doesn't quite sound like a knock, but its rhythmic with the engine, and you can hear it more pronounced
when you give it a little gas and coast back to a stop. After the truck has warmed up, sound is gone, and all is well.

2nd Issue, The transmission shifts great on normal throttle, but if i'm going 60MPH + and WOT. It will constantly shift in and out of overdrive, and will not find a happy place. My temporary fix, has been if i want to have fun to just shift out of OD manually.
any guidance on this would be great, as i'd rather not blow up my transmission by it constantly shifting back and forth. I've tried cleaning the MAF as I heard that could be a cause.

3rd Issue, When freeway driving and coasting 60+ MPH. after about 3 seconds of no gas and straight coasting, it feels like the engine goes into a compression and it slows me down. I also get a back fire when this transition happens and there is a noticeable audio change in the exhaust.

4th Issue, last month I changed out the tired injectors from the original engine, to the new style injectors, + distributor and my Ignition control module. in hops of fixing that noise I was hearing at startup. Only to make it so after the truck is warmed up it has a hard time starting. I have to give it a little gas or it will stumble on its face and die after 2 seconds. After it starts runs like normal.

Let me know if you need more information, or if i need to better explain something. Currently at work and trying to write this all on my break.
Thanks for any help!

 

Tapping noise- get a stethoscope. You can get a cheap one at Orilleys - tell us if the noise is in the top end or bottom. Does the noise increase in frequency with engine speed or road speed, or neither?

hard start when warm - check fuel pressure, and run codes. Specifically look for MAFS or map codes if equipped.

the transmission issues are out of my wheelhouse, unfortunately. Have you changed the fluid? If I had to make a guess I would say low/old fluid, clogged filter, or dying front pump. But remember, that’s a guess. And not a very educated one.

 

Hi Bio, Sorry it took took me a bit to get back to you been on vacation.
The noise seems to be coming up top in-between cylinder 3-5. This is a new long block less than 6k miles on it. Heads were reused from the original engine.
when reving the engine the noise is more pronounced but goes away when warmed up.
I'll check my fuel pressure I'm I believe i need to be at 60 PSI correct?
Only code I have at P1351. I've already changed the sensor under the distributor. I heard the other possibility is the Cam position sensor so I'll be changing that next.

I noticed while using torque pro app on my phone, that when I'm on the freeway and start coasting. My ignition timing goes from 35-40 to -10 -15 when off the gas.
It'll immediately pop back up as soon as i give it a bit of throttle. Could this be the cause of most of my issues?
thanks,