Engine Stalls when downshifting/coasting

Scott_S

Active Member
Jan 16, 2022
83
79
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Rock Hill, SC
New-to-me 1989 LX with all of the normal goodies. Unknown aluminum heads with roller rockers (or so I'm told, I haven't removed the valve covers myself, yet), unknown cam with a nice chop, Edelbrock Performer intake, BBK 70mm throttle body, K&N filter in stock airbox, BBK shorty headers, H-pipe, unknown mufflers (all 2.5"), 4.10 gears.
The car was off the road for 4 or 5 years until just before I got it. The place I bought it from installed new coil, distributor, wires and plugs.
I've really only put about 100-150 miles on it so far.

I recently did all the base idle reset stuff. Checked TPS voltage and it was within specs. Set timing to 14*. I currently have the idle set at ~850 RPM. I also topped off the old gas with 11+ gallons of 93 octane. All of that seemed to help a little bit.

I am experiencing a couple if issues that may or may not be related. I posted in another thread about having difficulty finding 2nd gear, but ONLY when downshifting. Going up through the gears is smooth as butter. Never grinds or pops out of gear. But, when coming down from 5th to 4th to 3rd to ....where is second? It doesn't grind, I just can't find it. The car has a quadrant and firewall adjuster. I will soon determine if the cable is adjustable or not, and install a Hurst shifter. I plan on a full clutch and shifter adjustment soon.
I've driven cars with bad synchros or gears and this doesn't feel like that.

That leads me to the stalling. When coasting or trying to find that 2nd gear, the car will often stall. It doesn't stall every time, but enough to make it an issue. It fires right back up, but it's super annoying.
The idle doesn't surge or anything. It's pretty steady, I think. I have not done any testing for vacuum leaks. I have not cleaned the MAF.

I thought maybe the clutch not fully disengaging might be causing it to stall, but it will sometimes stall when coasting or between other gears. Always when coasting or coming down from higher RPM's. Am I making sense?

One other thing: If I'm reading the tag right, the car was originally an automatic and is now a 5 speed. I do not know what computer is in it yet.

engine bay.webp
door tag.webp
 
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Have you checked for codes? The check engine light (CEL) only illuminates with an emissions related code so there can be codes and no CEL.

Have you checked the fuel pressure? Changed the fuel filter? Has it had a fuel pump replacement?

When swapping from an automatic to a standard the NGS can sometimes get missed.

 
- I have not checked for codes. I am very early in the "reviving it" stage. There is a tuner near me and I planned on taking the car there once I finished up refreshing everything.

- I have not checked for fuel pressure. I will soon have it in the shop to change all fluids (oil, brake fluid, power steering pump, probably coolant, etc.) and was going to ask them to check the fuel pressure.

- Fuel filter will be changed when I have the fluids done. I have a new filter sitting on the workbench.

- I do not know how old the fuel pump is or if it is stock or not.
 
2nd gear is usually the first gear to give problems and signals a " worn " transmission... Its the gear that gets upshifted and downshifted into the most..
 
2nd gear is usually the first gear to give problems and signals a " worn " transmission... Its the gear that gets upshifted and downshifted into the most..
Yeah, kinda aware of that. I'm really hoping that, because it's so smooth everywhere else, and when upsetting, that it's just an adjustment problem.

The new shifter is going in soon and I'll adjust it there. It will be going to the shop soon for fluids and I will have them check/adjust the clutch starting at the transmission since there's easy access on the luft.
 
So this is just me but if you are taking the car to a tuner with problems you need to talk with the tuner as some of them are not mechanics. This can get expensive and in a hurry.

Buy a code scanner and check for codes yourself. Checking the fuel pressure is easy and you can get a loaner kit at most of the parts stores. Changing a fuel filter is pretty easy.

If you are not a guy to turn wrenches I would question owning one of these cars unless you have semi deep pockets.
 
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I've turned a lot of wrenches in my day. I choose what to work on and what not to work on these days. Stuff like dropping the tank, changing the fuel filter, and accessing the clutch cable underneath are within my skill set. BUT ... my days of rolling around on the garage floor are over. I'd be working on my back, on a cement floor, on jack stands. I just don't want to.

I have a good shop that I use for general tune up stuff. It's way, WAY easier for them to do all that on the lift and I'm OK with paying them to do it.

This place is also near me; about 10 minutes up the road. My plan was to do, or have done, basic, general tune up stuff and then take it to them.

 
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Well, well, well .....
With the help of some suggestions from a Facebook group, I found out that my VSS is missing. I need to do a little investigating to determine what teeth are on the internal gear, but I can order and add the VSS. I'm assuming this is the correct plug, right?
Any reason why it would have been left out? The car has 4.10's, and I think the speedometer is pretty accurate currently.
If I check the indicated speed and it's correct, I just need to I.D. the gear in the tail shaft housing so I can get the right gear for the VSS, right?
speedo cable.webp
VSS plug.webp
 
Doing a repin on my LS1 engine I purchased for my " other " project....
Common to remove all of the wires and pins that go to the transmission ( Automatic and I will be putting a TKX behind it now) from the ECU... LT1swap.com, claims the ECU needs some signal from the VSS telling it that the car is moving......
I will be using my cable outlet to my speedometer and the electronic speedometer outlet for the VSS signal to my ECU... At least thats the plan ....
Sounds like you are on the right track
DSCF1654.webp
DSCF1653.webp
 
Well, well, well .....
With the help of some suggestions from a Facebook group, I found out that my VSS is missing. I need to do a little investigating to determine what teeth are on the internal gear, but I can order and add the VSS. I'm assuming this is the correct plug, right?
Any reason why it would have been left out? The car has 4.10's, and I think the speedometer is pretty accurate currently.
If I check the indicated speed and it's correct, I just need to I.D. the gear in the tail shaft housing so I can get the right gear for the VSS, right?

1989 5.0's should all have a VSS. Who knows why it was left off. I run into so many folks who misunderstand how these systems work and delete critical items because they don't think it's necessary. In this case the VSS provides a signal to the mass-air ECU that helps it recover idle when you come to a stop. Without it, as you've seen, you stall. Speed density ECU's only use the VSS for cruise control.

Anyway, i believe the speedo gear you already have will transfer over to the VSS once you get it. You will however need a new speedo cable as that one is not compatible with the VSS. My guess is previous owner may have ordered the wrong cable and said "well, i don't need the VSS".

And yes, that 2-pin plug hanging is the VSS plug.

No VSS signal generates a code 29 i believe. So if you dump codes now you'll likely see this code. Once installed, clear codes and redump to see if the code 29 has cleared.
 
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Yes. You don't need the connector (unless yours is bad). I'm 99% sure the speedo gear you already have will transfer over. When i converted my car to mass air I had to add the VSS and can remember only adding the VSS itself and changing the cable. Pretty sure I just moved the gear over