Trying to get my 91 LB9/G92 running right. Car is rock stock and has 66k original on the clock. No smoke, oil or coolant usage. When I got it a couple months ago, it ran very rough. We found and fixed two vacuum leaks - one by the fuel vapor cannister at the front driver's side of the engine bay and the other behind the HVAC panel.

Now, that smoothed things out quite a bit and really added power. But I don't think I'm out of the driveability woods just yet. I still feel a miss in the rev range. It feels like a choppiness and I notice it most between 1,500-2,000 RPM. At that range, holding your foot steady, you really feel a decided miss. It's bad enough so that the car wants to stall when I'm backing into the garage or into a parallel parking space and manipulating the clutch and engine speed in that 1,500-2,000 R range. It also feels a touch rough when you accelerate.

The prior owner apparently felt it too because maintenance records show they threw new plugs, wires, etc at it just a couple thousand miles ago. Obviously didn't do much beyond lighten his wallet a couple grand.

Help guys! Any input will be appreciated. It's particularly frustrating becaus I feel I'm a half step away from a perfectly running car, but something is just so subtly not right. Thanks!

 

FWIW, the previous owner replaced the following less than 3,000 miles ago:

Distributor Cap and Rotor
Spark Plugs
Spark Plug Wires
Coil
Ignition Module
Fuel Injectors

In other words, everything you'd replace as a knee-jerk reaction to a rough idle. I'm sure this $2,100 expenditure gave a nice psychological boost and he probably no longer felt the rough idle (you can convince yourself that anything's fixed if you spend enough money), I don't think any of these parts were the actual culprit, which is how I like to roll (find the actual problem). I fixed two vacuum leaks but even that didn't make the car run 100%.

 

I would first think injectors but since those are new or refurbished I would do a sweep on the tps.

http://www.youtube.com/watch?v=pTx4mKwUxJ4

 

Thanks. This would definitely fit the profile. When the throttle reaches a certain position, a bad TPS momentarily increases resistance versus continuing to open up, making the computer think there is less throttle than there is and, thus, providing less fuel. I'll pull it out and check it.

Anyone have any other ideas?

 

you can check the tps on the car but if you pull it unless someone replaced it with a early 85-89 tps it will not have elongated holes for adjustment.

http://www.iroczone.com/2009/10/adju...on-sensor-tps/

 

Got you. I don't have to pull it off to check then. Just insert the leads, turn the ignition to "on" and adjust the throttle to see if I get a smooth voltage increase. If I don't, the TPS is bad and should be replaced.

 

correct, a anlog vom shows glitches better but a digital one will work fine.

 

Ok, ran the test on voltage meter (analog) and the needle travelled smoothly and in sync with my movement of the throttle. No weird dead spots like in that first video. So I don't think this is the problem. Any other theories you think I could test out?

 

Ohm out your injectors, check your fuel preasure, then check for injector bleed down. Report back with your findings

 

Actually you read my mind. That was the next plan as this issue often is associated with injectors, and though mine are new, who knows. Will not get to until the week after next though. Will test and report.

 

What injectors did you swap in? Multec's or bosch III's? Also, who did you get them from?

 

I didn't swap any in. That was done by a previous owner in 2009, and there's no identifier on the receipt as to what brand they are or the source. I suppose I'll find out when I test them.

My guess is that he was chasing the same problem because all the parts one would suspect as the cause of a miss were all replaced at the same time. So I'm tending to doubt the fuel injectors are the culprit. But they need to be checked, because this problem is so closely associated with injectors. I mean, injectors aren't perfect, as you're implying and as others have said here. So it's entirely possible that he replaced a set with a bad one with a set with another bad one, if you know what I mean. I don't think it's likely, but it's possible, so it's important to rule this out so we can move on.

Adding to this, if I had to guess, they're just auto part store replacements. This car wasn't modded, and they were installed by what appears to be a typical independent mechanic, who we have to assume just sourced out the lowest cost ones he could with a reasonable level of quality.

 

I only asked because the multecs are known to be a problem. the bosch III's are much better

here is a side by side so you can identify what you have

 

Oh I see, thanks then. I'll double check when I get home but I believe they are the Multecs if these are the only two choices. The metal base is visible, and I doubt the small shop sprung for the best injectors. How much is a set of Bosch IIIs and where is the best source for the lowest price?

Verified -- They are Mutiecs. From my research, those are the factory ones, right? I'll run an ohm test on them next week and see if they're within specs.

 

just stumbled upon this thread. If you are interested in purchasing a set of bosch 3 injectors we can help you. Give us a call or shoot us a pm. Just an FYI, all tgo members receive a 10% discount with promo code tg11

 

you will want to buy them from him. Thank you Southbay08

 

Thanks much, I will look into doing this once I get under the hood next week and start testing everything. I note you offer a variety of Bosch IIIs ranging from 19 lbs to 42lbs. Which ones are recommended for a rock stock car that one has no intention of modifying? And what PSI are the stock injectors from the factory on a '91 LB9/M5?

 

FWIW, it has also been suggested that I check the EGR by unplugging it. I'll add that to the Ohm test of the injectors and report the findings. I am going to find this gremlin!

So far, to keep score, we've fixed two vacuum leaks which improved things but did not totally cure the problem. We've also ruled out the throttle position sensor as that gave a consistent progressive voltage and resistance reading.

 

19#hr at 43.5 psi

http://www.southbayfuelinjectors.com...7&product=1500

 

More symptoms to report. Car got 15.5 in mixed city/highway after a 100 mile loop. I think this is off by 2-3 MPG as the 305 TPI is rated at 15 mpg city and 24 mpg highway.

Car has trouble restarting on hot days after I've been driving awhile. On a recent 95 degree day, it wouldn't turn over at all (I think it overheated the starter solenoid) until it cooled down.

It always runs just north of 220 when I get into traffic on warmer days. At that point, the driver's side fan kicks on and struggles to keep the temp at that level. You shut it off on a hot day and that gauge can get near the red, until it fires up and goes back down to 220 with the fan.

I ran the injector OHM test today. They all tested at about 13 ohms. Since they're all the same, I doubt injectors are the problem. My search of the board shows anything over 12 is good, correct?

So I yanked off my plenum to pull my EGR, as I'm starting to become very suspicious of that part. After I got the plenum off, I noticed that the hard vacuum line that runs from the EGR solenoid to the throttle body was not connected and was just hanging there (down below near the solenoid itself).

If I didn't do that accidentially when I pulled the plenum off, that means I had a third vacuum leak, and it would follow that this would screw up the operation of the EGR.

FWIW, I also noticed that the rubber caps under the throttle body are dried and cracked. One clearly was not sealing anything it was so bad. Vacuum leak #4 anyone? LOL!

I noticed no carbon under the valve and the diaphram seems to move back and forth freely. It's the original Delco unit, PN 17090107. Next step will be to hook a vacuum pump to it and see if it holds vacuum. If so, I'll bolt everything back together with new gaskets, replace the throttle body's rubber caps, and see what happens.

 

just a quick question regarding the injectors, did you ohm them when they were cold and hot?

 

Cold

 

do yourself a favor, check the injectors when they are hot as well. The coils have a tendency to shut down and fail when they are hot. They are borderline at 13ohm cold.

 

I will definitely do this if my EGR work doesn't solve the problem. Can't heat up anything right now as the plenum is off the motor!

 

Found the problem! As I said, plugs, wires, distributor cap, rotor, ignition coil, and fuel injectors were all replaced a few thousand miles ago by the prior owner. I then checked throttle position sensor, fuel injector Ohms, replaced the EGR valve, replaced the EGR solenoid, fixed three vacuum leaks, checked the plug wires for proper sequence, and even replaced a cap and rotor myself as a terminal came out of the “new” cap when I pulled a plug wire off. Obviously, the upper plenum gaskets and throttle body gaskets were replaced during this. I sprayed carburetor cleaner on all vacuum connections. I turned out the lights in the garage and looked for spark arc. I even prayed over the car once.

The car ran identically to before. I brought it to my friend’s repair shop. He drove it around, inspected it, and said nothing is wrong with it. He said it’s one of the most powerful stock Third Gens he’s ever driven that pulls strong all the way through the rev range (he owned one also). I concur with this, as the car really does feel strong, much more so than you’d expect from a 305. He thought the several German and Japanese cars I’ve owned since my last Third Gen had spoiled me. In other words, I unintentionally became a car snob, and a 1980s V-8 is setting off my internal “check engine” light!

Could be so. Dejected and annoyed, I suddenly remembered that when I was changing my transmission fluid, the transmission mount was shiny and new looking, which struck me as interesting. I'd quickly disregarded it then. Now, my hand noticed how much that shifter always vibrates. Then, I thought of the strange harmonic vibration noises that the car makes at highways speeds sometimes, that I’d previously written off to a Camaro being a Camaro.

Then the light bulb went off. I crawled under the car as far as I could and sure enough, the words “Energy” were visible on the front of the mount.

A quick Google search revealed that Energy Suspensions makes polyurethane transmission mounts. Type in “polyurethane transmission mount vibration” into Google and you’ll get a ton of hits. The first one will be from Energy’s website itself, telling you polyurethane mounts may increase vibration. Duh!!!!! Might as well drill a hole in a cinderblock and stick in under there.

So I went to the dealer and got a stock Delco rubber transmission mount and installed it. Lo and behold, vibration is completely gone and car feels smooth – well, it’s no E36 M3 with the S52 inline six but at least it no longer feels as though something’s wrong with it.

In sum, at some point, someone put this worthless “performance” mount in there, no doubt because of all the message board wives tales that rubber mounts cause “horsepower loss” LOL!! That, in turn, transmitted the natural V-8 vibrations directly into the car’s unibody, making subsequent drivers, including myself, think the car was missing or otherwise out of whack.

One spent over $2,000 in the Part Toss Olympics. I spent a couple hundred and frustration myself. I’m glad it’s over but wow. I mean, just wow.

Moral – Don’t install Polyurethane motor and transmission mounts in these cars unless you want it to feel like a UPS truck. Unless you have a 1,000 horsepower motor in it and use it for bracket racing, you don’t need them. And if you do, you need solid mounts. Thanks for the input everyone.

 

Just read through all this and can't help but laugh. It's amazing how something that simple could cause all that. I was just reading something the other day about those Energy trans mounts being a real big pain. I'm all about stock when it comes to a majority of things which is why my 87 GTA is just that. Once you start putting on aftermarket this and that is when you start having trouble.

Either way, glad you got it all figured out! Now time to enjoy the open road

 

Thanks man. Good news was I got to know the car quite a bit better trying to find out what in hell was wrong with it. I was already no fan of most aftermarket parts. Often they're snake oil and unless you engineer everything together just perfectly, you usually negatively impact driveability and quality of life with the car, and sometimes won't even make it faster.

Fortunately, mine is pretty much all stock, saving for said concrete transmission mount and a set of K&N air filters. I'm also going to toss those and get some regular paper ones. I'll some how have to live with the 50 horsepower loss that going with mere paper will cause

 

Here's a story of a young gentleman from the rival camp who had the same problem ..

http://forums.corral.net/forums/94-9...vibration.html

The responses range from "damper probably goin'!" to "get an aluminum driveshaft" to the absolutely brilliant "install cement engine mounts too, that'll balance her out for ya." Logic behind that one is you can install the polyurethane engine mounts with a stock transmission mount but not vice versa because..... well, just because! Another opined that certain model years are more susceptable to vibrations from the mounts than other years. Good lord. See also...

http://www.ls1tech.com/forums/automa...ibrations.html