DIY tech tip: if your 3800 V6 Buick or other GM car is shifting hard into reverse or drive after cold starts, check the line to your transmission vacuum modulator valve | Enthusiast 101

If you’re a regular AutoNewsblaster reader, you might have already noted that my current daily driver is a 1995 Buick Riviera with a supercharged 3800 V6 (3800 being GM’s brand name for several very successful generations of 3.8 liter V6 engines that originated in the Buick division). I replaced the Riviera’s transmission vacuum modulator valve a couple of years ago. Replacing it did no harm, although it might not have been a necessary repair. But recently I experienced a sudden and perplexing issue that turned out to be related to that valve.

First, some background. I’ve owned the ’95 Riviera for four years now. I love it to death and overall it has been remarkably reliable, especially considering that it’s now a 25-year-old vehicle.

But this overwhelmingly very positive ownership experience comes in spite of some maddening quirks of the Series 1 Buick 3800 V6 (1991–early 1995, with the “Series 1” designation being somewhat deceptive because it is in fact not the first generation of the 3800), such as a number of plastic gaskets that make the motor rather prone to leaks of oil, coolant, and other fluids.

I’ve written about those quirks in a past article that takes an in-depth look at the Buick 3800, so I won’t go into great detail about them here. 

All that said, after investing a total of about $6,000 in repairs in the Riviera  over the previous three years, 2019 ended up being my most trouble-free and inexpensive year of ownership. The issue that led me to replace the transmission vacuum modulator valve—a cheap and easy DIY repair—emerged within my first year with the car.

It began when I started to notice what felt like rough shifts at seemingly incorrect moments when the engine was under load, such as when I would start to throttle back as I approached the crest of a hill I was climbing at a fairly high speed. The sensation was that entire front-wheel-drive powertrain was jumping, as if trying to break free from its mounts.

Sometimes it would only jump once, other times it would have a few spasms over the course of a few seconds. Either way, it would usually smooth out on its own, and there was usually a fairly long interval between these jumping episodes. Some days it would jump, other days it wouldn’t. At times there seemed to be a loose correlation with colder weather, but as time went on the issue seemed increasingly random.

The existence of a part called the transmission vacuum modulator valve was something that I stumbled over, while flipping through a Fram catalog in an auto parts store, while I was looking for the part number for a replacement air filter.

Interestingly, the Fram catalog had transmission vacuum modulator valves in the catalog with consumable items like air filters and oil filters even though, to my knowledge, the valve isn’t a routine maintenance item. I didn’t buy the valve on the spot, but it crossed my mind that it might be related in some way to what felt to me like a shifting problem.  

Meanwhile, several shops that I had check the car out over a period of a few months did not find anything conclusive about what was creating the occasional sensation of rough, jumping shifts. The ECM wasn’t throwing any codes.

But an even bigger complication was that the occurrence of the jumping was entirely unpredictable. Techs who road tested the car weren’t able to replicate the problem—nor was I able to replicate it on a couple of occasions when I took a tech for a ride with me.

However, through some research on forums, I found that some owners of 3800-equipped vehicles ranging from other Buick Rivieras to Chevy Camaros of mid-to-late 90s vintage were describing seemingly similar hard-shifting scenarios that were resolved by changing out the transmission vacuum modulator valve.

When I dug a little deeper, it seemed to make sense. Engine load is one of the determinants of when a transmission should shift. When you’re climbing a hill, the load is higher, and when the load reaches a certain point, it’s optimal to shift into a lower gear. Conversely, when you’re cruising gently down a flat stretch of road, or coasting downhill, the load is lower, so a higher gear is fine.

The job of the transmission vacuum modulator is to provide one of the “information inputs” that tells the transmission how much load the engine is under and when it needs to shift. Through a vacuum connection to the throttle body, the valve measures the level of vacuum from the engine, which is an indicator of how hard the engine is working—how much load it’s under, in other words. It therefore seemed reasonable that a valve that was faulty and not measuring vacuum accurately could be resulting in incorrect shifts.

So I decided to give it a shot and replaced the valve, on a day when I happened to be taking the Riv on a three-day roadtrip to a destination about 150 miles away. On the open highway, the eighth-generation Buick Riviera is truly in its element, and the cruising felt smooth as silk for the entire roadtrip and for several weeks afterward. I thought I had finally solved my mysterious powertrain-jumping problem.

But it eventually came back, revealing that something else was amiss, even though I could take comfort in knowing that I at least had a new transmission vacuum modulator valve.

Meanwhile, months later, the ECM finally started throwing codes for something quite different from what I would have expected. The jumping problem had become a good bit worse and more frequent, but it was still maddeningly elusive when it came to trying to have someone diagnose it.

Finally it started happening with enough magnitude and frequency to trigger the Check Engine light, even though at first the light would reset and not turn on again the next time I would start the car.

The situation came to a head one evening when I was accelerating hard out of an intersection. I felt a big jump while the four-speed automatic was still running through the drive gears. It hadn’t even shifted into fourth before the car stalled entirely. Fortunately, when it stalled I was just shy of an entrance to a huge parking lot of a local factory, so I was able to coast into the lot and park the car. I couldn’t re-start it, so I called AAA and started waiting for a tow to a local Chevrolet dealer, the closest shop I knew of that had the best mix of equipment and know-how to diagnose pesky problems on a grunge-era Buick.

As it turned out, the AAA dispatcher called me back and told me that the wait for a tow truck would be pretty long. So I tried one last time to restart the Riv, and this time it was successful. I took it slow, and the jumping recurred a few times. But I was able to make the trek of about four miles to the Chevy dealer and drop the keys with a note for them to call the next morning.

The team of techs at the Chevy dealer was mostly of the millennial generation, but they apparently had at least one older dude on staff who was working at GM dealers back in the 90s, when both Pearl Jam and the Buick 3800 V6 engine were still in their heyday.

The Generation X tech also had difficulty replicating the problem, but the ECM had finally stored codes seemingly related to the issue: camshaft–crankshaft correlation error codes. And by the way I described the problem as more apt to occur when the motor was at full operating temperature and under load, the tech’s instincts directed him to the crank sensor.

So he replaced the crank sensor. That was about two years ago, the car has been entirely free ever since of the problem that misleadingly felt like the transmission was jumping gears. And the car has also been relatively free of problems overall. Just about everything I’ve spent on the Riviera ever since has been on expected wear-out items like brakes, shocks and struts, tires, etc.

But meanwhile, I had this relatively new transmission vacuum modulator that I didn’t think about very often. Thinking back to when I changed it, though, there were a couple of head-scratching issues.

When you search for these parts online, you usually see them pictured with a segment of rubber vacuum line and an O-ring, suggesting that these are parts you should replace along with the modulator valve when you change it out. Mine, which I was able to find at a local parts counter, came with the O-ring, but not the short vacuum hose segment, which connects to a much more durable narrow, solid plastic vacuum pipe that leads back to the throttle body as a vacuum source.

I didn’t think too much of it at the time. The existing vacuum hose segment did feel a bit dry-rotty on the outside, but I didn’t see any evidence of holes or cracks that went all the way through. So I just went ahead and used the old vacuum hose when I changed out the valve. At the time it seemed fine, so I didn’t give the dry-rotting vacuum hose much thought for a very long time.

Fast forward to 2019. In the last quarter of the year, I made a couple of minor DIY repairs. The first one was the upper radiator hose. The old one sprung a leak about 12 miles from home one Sunday afternoon. When I saw the steam, I was luckily just a block away from an Advance Auto Parts store, so I pulled in with the intention of buying a hose and making an emergency redneck repair, changing it out in the parking lot.

The exact-fit, custom molded replacement was not in stock and would have taken a couple of days to special order, and in my circumstances I didn’t have a couple of days to wait. So I bought a two-foot generic flexible coolant hose, the kind in which the rubber is reinforced with wound wire.

Because it wasn’t a custom fit for the layout of my engine compartment, it was a bit awkward snake the generic hose into place in a way that wouldn’t interfere with the maze of other parts, wires, and lines in the front of the engine compartment, near the radiator, but I didn’t note that I’d knocked anything out of place as I installed it, and I was able to make it fit and do its job of connecting the thermostat housing to the radiator upper radiator inlet.

Everything still seemed copacetic for a couple of months after I changed the radiator hose. Then, about two months later, the passenger-side headlight burned out. The bulbs, which where the same ones that were in the car when I bought it four years ago, were cheap and on the dull side, so I replaced them both with the brightest halogen bulb that Sylvania makes.

On the eighth-generation Buick Riviera, the headlight bulbs are maddeningly difficult to find and reach—probably easier to access from underneath when the car is on a lift.

Although it’s a tight squeeze on the passenger side, as long as your arms and hands aren’t too ginormous, you can reach down from above, just behind the radiator cap, and get to the sockets for the separate high beam and low beam bulbs.

But on the driver side, it’s another story. The high-beam bulb is a bear to get to, and unless you have very small arms and hands it’s a challenge to reach it without taking off a brace that helps secure the side of the radiator to the inner side of the front fender.

Fortunately, I only needed to change the low-beam bulb. Buy even that bulb, frustratingly, is almost impossible to get to without taking off the air cleaner housing. It isn’t that big of a deal to take the intake off, and I’ve done it a number of times for DIY air filter changes, so that’s what I did.

The area just behind the headlights on the front driver side of the engine compartment is tight, and the air cleaner housing is very close to where that new upper radiator hose was looping down near—guess what—the vacuum hose that leads to the transmission vacuum modulator valve.

As I removed and reinstalled the air cleaner housing in the course of changing the low-beam headlight, I didn’t pay much attention to whether I was coming into contact with the new radiator hose. And the transmission vacuum modulator hose wasn’t even on my radar at all.

That’s why it took me a while to make a connection maybe a week or so later when a possible transmission issue did pop up on my radar. During a cold start on a particularly cold morning, I noticed that the transmission shifted harder than I ever remembered it shifting before into Reverse, and then into Drive.

Great, I thought to myself. What’s going on now? Once I started town the road, though, the shifts through the drive gears seemed fine. And I also noticed that, once the engine was at full operating temperature, the roughness when shifting from Park to Reverse and Reverse to Drive subsided.  But on every cold start, the harshness would be back.

I was perplexed. How had this rough shifting just come out of nowhere? A number of theories ran through my mind.

For instance: once quirk that I had always noticed about the car, but never had a tech look into, is that the idle speed would increase when I would turn the headlights on, as if the ECM was responding to an increased electrical load by upping RPMs to get more juice from the alternator to keep the battery from losing charge.

I don’t know whether that’s the true explanation for the increased idle speed, but the hypothesis made sense, and I extended it to wonder whether an electrical load increased even further by brighter headlights might be pushing the idle up too high and contributing to the rough shifts on cold starts.

The effect of headlights on the idle speed was also something that seemed to subside at full operating temperature, which seemed consistent with the pattern of the rough shifts. But once it became clear that the rough shifts on cold starts seemed to happen regardless of whether the headlights were on or off, I dismissed the theory.

But the idea of some kind of correlation with the timing of the headlight change continued to gnaw away at me.  What could have happened around the time I changed those headlights to also cause the rough shifts?

Then, just about a week ago, the lines of thought all converged. I thought through what my process was as I changed the headlight on the passenger side. I remembered how it was a struggle to access the bulbs, and how I’d had to pull the air cleaner housing off. What else was there that I might have disturbed?

And then I remembered the vacuum modulator valve, which I’d given hardly a thought for the couple of years since my jumping powertrain problem had resolved.

The vacuum modulator valve. Of course! Let’s take a look! I popped the hood and eyeballed the valve. It was more obvious than I could have imagined. On the tube in the front of the valve where the vacuum hose slips on, there was a small segment of hose about half a centimeter long, and that was it—as if it the rest of the hose had been cleanly sheared off.

WTF? Where was the rest of the vacuum hose? And where was the solid plastic pipe that connects that vacuum line back to the throttle body?

I nosed around a bit and found that the vacuum pipe resting against the front valve cover. When connected, it slants forward toward the front of the engine. Although it was detached from the vacuum modulator valve, it was otherwise intact and properly connected to the throttle body on the other end. 

At the end that connects to the transmission vacuum modulator valve, there was still enough vacuum hose left to reconnect it, and that’s what I did. Unsurprisingly, it was pretty dry-rotted, so I knew I’d better replace that segment soon.

But in the meantime, it was good enough to do the job. I let the engine cool down, started it cold, and shifted through all the gears at idle with the brakes applied. The shifts were as smooth and fluid as I ever remembered.

Problem solved, apparently.

So the takeaway? If you’re experiencing hard initial shifts into gear at idle after a cold start and happen to have a Buick 3800 or one of the many other engines from a variety of automakers that has a transmission vacuum modulator valve, check the state of that valve and the vacuum lines connected to it.

If that turns out to be the culprit, it’s a pretty easy fix.

Meanwhile, if you’re going to tackle this repair a DIY job on a 3800 or another engine with a similar architecture with regard to the transmission vacuum modulator valve, here are some tips.

DIY transmission vacuum modulator valve replacement tips.

• My tips here are brief, general, and big-picture, NOT a complete, step-by-step repair procedure. The correct replacement procedure will vary with the year, make, and model of your car. Get hold of a full factory service manual that will detail the complete, correct procedure. For many cars, PDF versions of factory shop manuals are available online, such as on Amazon, for pretty cheap.  We disclaim any responsibility for undesirable outcomes, damage, or injury. So go into this at your own risk, with a solid understanding of what you are doing.
   
• Don’t be confused if the replacement valve you get looks quite a bit different from the original. The original transmission vacuum modulator valve may be bulbous and painted black, but the replacement you get will likely be conical and brassy. If what’s currently in the bellhousing is bulbous, that’s a strong clue that your valve has never been replaced. As long as you’ve looked up the correct replacement part for your exact year, make, model, and engine, you should be fine, even if the replacement looks entirely different.
  
• Look for a replacement part that comes with a new O-ring and a new segment of vacuum hose (it’s just a couple of inches long). Mine did not come with either, or perhaps I ended up buying a part that someone had returned to the parts store without the O-ring and vacuum hose. Because even the last generation (2003–2008) of the Buick 3800 V6 is starting to get some age on it, the existing segment of rubber hose that connects the valve to the solid plastic vacuum line is likely to be dry-rotted. If you MUST use a valve that doesn’t come with a segment of vacuum line, buy an appropriately sized segment of hose to replace the old one that’s currently connected to your old valve.
  
• If at all possible, make the repair with the engine cold. If you must do it with the engine hot, wear a pair of work gloves that will give you good protection from burns while also giving you enough dexterity to twist wrenches, detach and attach vacuum lines, and generally maneuver in tight spaces. The transmission vacuum modulator valve is fairly close to the exhaust manifold, which get’s extremely hot once the engine has warmed up. And, while you can probably work on it without touching the manifold directly, it is in a crowded space surrounded by other metal parts and lines that pick up heat from the manifold. In the three times that I’ve worked in this area of my Riviera’s engine compartment, I’ve singed the skin on the back of my hand from just a brief, fraction-of-a-second graze of one of those metal parts. It doesn’t take much to get a most unpleasant, blistered burn. So do the job with the engine cold or wear the right gloves.
  
• The transmission vacuum modulator valve looks like you can just pull it right out of the bellhousing and pop in the new one, but more than likely it’s secured by a bracket, attached to the bellhousing with a single bolt that mates with a flange under the conical valve body. Cars that had stock 3800s are pretty old now, and the bracket may be easy to miss because it’s likely to be covered with some layers darkened grease, muck, and road grime.
  
• Before you remove the old transmission vacuum modulator valve, clean old the exposed area at the front of the old valve and the surrounding area with an appropriate engine or parts cleaning product. Spray the cleaner onto a cloth and wipe the part and the surrounding area to clean off any grime. Don’t spray it directly onto the engine, transmission bellhousing, or anywhere else in the engine compartment. These cleaners can be flammable, which is another good reason to do this job with the engine cold. Then, when you remove the old part, clean the area surrounding the bore in the bellhousing into which you’ll insert the new part. All this cleaning is to prevent foreign grit, grime, and dirt from getting inside your transmission. Trust me, you don’t want that.
  
• When you pull out the old transmission vacuum modulator valve, the O-ring may stay lodged in a flange at the top of the bore where the valve enters the transmission bellhousing. Remove this O-ring VERY carefully—you don’t want to drop it inside. Once again, follow the procedures in your factory service manual, but the good news here is that you probably don’t need to re-seat the new O-ring into the flange where the old one was lodged. The O-ring just slides over the shaft that inserts into the bellhousing, butted up against the bottom of the valve body. When you insert the new valve, the O-ring should then mate properly with the flange and do its job of keeping transmission fluid from leaking out. But once you’ve completed the replacement, warm the engine and shift through the gears, and then check for leaks just in case.