1991 Hardbody SE-V6 - runs but just barely

[SEV6Reborn]

Hi, all. New to the forum, but not new to Nissan. I sold them years ago and they're all I've driven since. My main ride is a 2008 Titan.

But I also have a 1991 SE-V6 4x4, with the VG-30E engine. It has served me well over the years, until now. It sat for a few years in my garage, and it's my own fault for not driving it more often and keeping it in shape. I recently put a new battery in and was going to get it in shape for the Winter, in case we had any treacherous weather.

Well, it starts and runs, but the idle is absolutely horrible, it shakes, and has practically NO power. The idle smooths out at around 2800 RPM, and sounds and feels normal above that. But it won't rev past about 4000 RPM.

I've taken it on the highway twice and it'll barely get to 25 mph downhill. Hoping for some help here.

This is what I've done so far:

New plugs and wires
New coil
New (rebuilt) MAF sensor, but first tried cleaning the old one
New PVC valve
Ran Seafoam to clean the injectors
Flushed the old gas and put in fresh non-ethanol
New fuel filter
New air filter

Nothing has made any difference. Not being a mechanic with diagnostic skills, I've just been throwing parts at it, sad to say.

Next up might be the EGR valve, but I'm having trouble figuring out exactly where it's located (behind the intake manifold, maybe?). I've also considered an exhaust restriction. Stuck in failsafe mode, maybe?

I pulled the passenger seat a couple of weeks ago to check the computer diagnosis. There are no stored codes, and Mode 5, real-time diagnosis, tells me nothing.

To summarize...

Barely runs
Very rough idle
Shakes
Won't get above about 25 MPH downhill
Idle smooths out at 2800 RPM
Won't rev above about 4000 RPM (redline is about 5800)

Any help is appreciated!

[Niti QX4]

Sounds like a vacuum leak in an old hose. Check on YT for DIY smoke "machines" so you can smoke the lines. Please keep us updated on what you find.

[SEV6Reborn]

I've done the trick where I spray carb cleaner at every vacuum joint I can access, and found nothing so far. I might see if smoke machines can be rented.

[VStar650CL]

I once had a "mystery" vacuum leak show up in my bay which turned out to be a pinhole in the vacuum booster diaphragm. The STFT clearly showed there was a leak, but probably four other guys looked at it before I did with a variety of methods and found nothing. Smoke showing up in the driver's floorboard was what located it. There's no substitute.

[SEV6Reborn]

It won't surprise me a bit if there's a leak, or more than one. I tried the trick where you hook a hose to a vacuum port on the intake and blow cigarette smoke across it. That did no good.

Honestly, it acts very much like a former truck I had that had a clogged catalytic converter. That's why I'm leaning toward it being an exhaust restriction or clogged/bad EGR valve.

[VStar650CL]

Exhaust clogs (unless they're total) don't usually affect the idle much. With a typical partial blockage it will seem fine at idle and light load, but as the RPM's rise the manifold vacuum drops to nothing and the engine will make a very distinctive howling or moaning sound. If it isn't howling under heavy pedal, chances are the problem isn't the exhaust.

[SEV6Reborn]

Exhaust clogs (unless they're total) don't usually affect the idle much. With a typical partial blockage it will seem fine at idle and light load, but as the RPM's rise the manifold vacuum drops to nothing and the engine will make a very distinctive howling or moaning sound. If it isn't howling under heavy pedal, chances are the problem isn't the exhaust.

Good to know, and that makes sense. Thanks.

[Niti QX4]

Thanks for reporting back. Vacuum leaks can occur at ANY point that sees vacuum, not just the junctions. As VStar650CL wrote, it could be your vacuum booster, or anywhere else too. There is no substitute for using a smoke machine for finding stubborn vacuum leaks.

The smoke has to put into the intake system under light pressure to "show" the leak. It's not hard, but most seem turned away by the use of a "new" tool. There are DIY ones made with a mason jar, cigar, paper soaked in oil, you name it (and very inexpensive to make - I made mine years ago with a Halloween "fogger").

Also, I would mention to keep your original OEM parts and not throw them away. I have found over the years that putting more parts on will "hide" the original problem by adding more problems.

[SEV6Reborn]

Thanks for reporting back. Vacuum leaks can occur at ANY point that sees vacuum, not just the junctions. As VStar650CL wrote, it could be your vacuum booster, or anywhere else too. There is no substitute for using a smoke machine for finding stubborn vacuum leaks.

The smoke has to put into the intake system under light pressure to "show" the leak. It's not hard, but most seem turned away by the use of a "new" tool. There are DIY ones made with a mason jar, cigar, paper soaked in oil, you name it (and very inexpensive to make - I made mine years ago with a Halloween "fogger").

Also, I would mention to keep your original OEM parts and not throw them away. I have found over the years that putting more parts on will "hide" the original problem by adding more problems.

There are some smoke machines on Amazon in the ~$70~ range that get good reviews, so I might go that route.

[Niti QX4]

Great! Please keep us updated.

[SEV6Reborn]

Okay, the smoke machine arrived Friday morning, and I'm very pleased with it (aside from the stench of burning baby oil). I found a steady stream of smoke coming off the rear of the intake. I plugged that 1/4-inch hose back in and it took care of that one.

Then I found two more: the hose on the PCV valve, and the one that attaches to the booster. I replaced them both today (not factory OEM, of course), ran the smoke test again, and found ZERO leaks this time. Great news, right?

Nope. Now instead of idling rough and having no power, the truck won't run at all. It'll sputter and "run" for maybe two seconds at about 300 RPM, then die.

So I must ask, what does that tell you? I fix the vacuum leaks, now it won't run. Maybe tomorrow I'll momentarily put the PCV hose with the crack back on just to see what it does. If it runs with that back on, what would that tell you?

[SEV6Reborn]

By the way, if I need to check the AAC valve, is it even possible to get it out of there without removing the intake manifold?

[Niti QX4]

First: CONGRATULATIONS!! You fixed the hardest thing for most people to fix. Vacuum leaks stump everyone without a smoke machine. You did an awesome job!

Second: Put back on the what old components that are good you can still use. OEM MAF for sure, as long as it works. Put it back on first and change nothing else. Try to start and run with just that ONE CHANGE.

PCV should rattle if you shake it. The thought here is to have as few things changed as possible to minimize compounded failures. A decent amount of "new" repair parts simply are no good out of the box. Hard to know what to leave in and what to leave out (sounds like an old song).

Third: If you want to leave on some of the new items (and I don't blame you), I would recommend starting from scratch on all your idle settings following the factory service manual (FSM) provided by this site.

Please keep us updated. Thanks, and congratulations again!

[SEV6Reborn]

Thanks. I'm going to to some experimenting today. I have the original MAF and EGR sensors. I tossed the PCV valve, but the new one did indeed rattle. I think my first step is to just pop the vacuum hose off of the PCV.

If that causes it to run, what will that tell me?

[SEV6Reborn]

Well, no dice. The only thing that has changed since it ran last is swapping those two hoses. I put them both back and it still won't start. It threatens to, but just can't quite.

Honestly, I'm in over my head and it's going to a shop at some point soon. I've thrown parts at it and bought tools, and I've probably already spent more than it would have cost to have sent it to a shop in the first place. I appreciate the help, but I can't solve this mystery.

I never set out to learn to be a mechanic. I just wanted my old truck to run right again, something it had always done until now. So it's time to turn it over to a pro who can probably figure it out in ten minutes.

[Niti QX4]

Don't sell yourself short, you did a great job! At this point, you already know more than most when it comes to fixing your own vehicle. If someone tells you it's a vacuum leak, you know you have already tested for that and fixed those issues.

I can't speak to your area, but diagnostics here are 285.00 per hour at the dealership. Independent shop is less, but usually around 150.00 to 175.00 per hour (one hour minimum).

Parts will usually cost about double what you would pay for them (shop needs to make money). So, you have saved yourself a good amount of money already.

FAST: Fuel, air, spark, timing. You need all four to run an engine. You have fixed the "air" part by closing up your vacuum leaks. To check for fuel issues, all you need is a can of starter fluid to spray into the throttle body and see if the engines "fires". If it does, you have a fuel issue.

I would check the idle set points first and make sure it is set right per the FSM.

If you decide to take it to a shop, you did your best and that's awesome! Again, congratulations on even trying. Many wouldn't even start to put in the effort.

[SEV6Reborn]

Thanks! I have a certain theory going on here. It ran, but roughly, until I replaced the two leaking vacuum hoses. Now it won't start at all. This leads me to to think that maybe those two leaks were the only places it was getting enough air to idle. So now that they've been shut off, it gets none.

So it looks like I need to pull the intake manifold and see if the AAC valve is stuck.

What do you think?

[Niti QX4]

Makes sense. I would think someone would’ve adjusted the idle to compensate for the extra air

[Niti QX4]

OP, any updates?

[SEV6Reborn]

OP, any updates?

Sort of. After replacing the hoses with the leaks, it would barely run at all for a few seconds. Then it died and hasn't started since. I even tried putting the old hoses back on and it made no difference.

A mechanic friend is going to look at it on Saturday. He seems to think it's the AAC valve. That makes sense if the leaky vacuum hoses were the only place it was getting air to just barely run.

He's also going to confirm that the timing is good and some other things.

[Niti QX4]

Awesome! Please keep us updated. Thanks!

[SEV6Reborn]

I have an update. My mechanic friend went over it on Saturday and we eliminated several things. The timing is perfect and the belt looks very fresh and undamaged. Fuel pressure is good. There's also no exhaust restriction.

But we finally know the answer. Only two cylinders out of six were firing. Not kidding. I sat there and held the pedal at about 1000 RPM's while he carefully pulled each plug wire with insulated pliers. Only cylinder #1 and #6 made any difference at all. He measured the resistance on the three injectors on the passenger side which we could access. Cylinder #1 gave a reading that was slightly out of spec, while #3 was way out of range and #5 showed infinite, meaning no connection.

So I have a whole set of injectors coming in on Wednesday. I also have gaskets for the intake plenum and AAC valve, and he's getting gaskets for the valve covers, since they have a little seepage and we may as well do them while the plenum is off.

[Niti QX4]

Awesome news, thank you for sharing that with us - lad to hear the repair is almost complete! Please keep us updated. Thanks!

[SEV6Reborn]

Awesome news, thank you for sharing that with us - lad to hear the repair is almost complete! Please keep us updated. Thanks!

Thanks. We're installing the new (reconditioned) injectors on Sunday, along with valve cover gaskets while the plenum is off.

[Niti QX4]

Looking forward to Sunday to hear how it goes!

[SEV6Reborn]

Update: We worked on it for many hours yesterday. Removed the plenum, swapped the valve cover gaskets, checked out the AAC and it was fine, swapped the injectors, then put it all back together.

It fired up and ran like new... for about two seconds. Then a miss appeared. It felt like it felt the last time I was actually driving it a few years ago.

Cylinders #3 and #5 are dead. He could barely hear any "tick" at all from the injector on #3. The injector on #5 sounded normal, but it (and maybe both of them) was sending raw gas out of the exhaust.

Our theory is that the #3 injector is dead and #5 is stuck wide open, resulting in both cylinders filling with gas and fouling the plugs.

The reviews on these remanufactured injectors are iffy at best. I keep seeing comments like, "Only three of the six work" and so on. And individual ones are nearly as expensive as this set of six I bought.

So... since four of the six worked, I've ordered the same set of six again, because surely at least two out of the six will be good. And of course, new plugs for #3 and #5 since I'm pretty sure those two are fouled.

[Niti QX4]

Outstanding! You are almost there, and you did it yourself!

[FrenchPople]

Something that's a little off and I'll go ahead and chime in here, by chance have you considered getting a factory service manual? Understanding it is an OBD1 vehicle, you can still go through a symptoms and troubleshooting guide through the EF and EC section of the factory service manual in order to diagnose all sorts of different issues like what you have. It even has page numbers to direct you to in order to test certain parts.

[Niti QX4]

OP, please update us on when the new injectors come in and are installed. Very much pulling for you and hoping you get at least two good injectors to get back up and running.

[SEV6Reborn]

Something that's a little off and I'll go ahead and chime in here, by chance have you considered getting a factory service manual? Understanding it is an OBD1 vehicle, you can still go through a symptoms and troubleshooting guide through the EF and EC section of the factory service manual in order to diagnose all sorts of different issues like what you have. It even has page numbers to direct you to in order to test certain parts.

I have a copy of the manual from here. One thing I hate about it is the fact that it was scanned as images instead of OCR, so you can't search text.

I've referred to it several times, but it feels much better to work with someone who knows what they're doing and has the tools.