mikenj73

Hey guys, my 1991 3.1 Camaro started running rough right after pulling away from a red light the other day and i cant figure out what is wrong with it. Basically it just started acting like it had a random miss here and there. At the next light, the RPMS would just drop for a second at a time, then return to normal. It doesn't want to accelerate when i step on the gas. In neutral, it will bog down if you floor it. You can still drive it around town, but it takes a while to get up to highway speed. Even then, it doesn't want to go much more than 65 or 70. At first I thought it might have been the gas, since it was almost on Empty when it happened. Just to be safe, i filled it up with high test and put some HEET in there just in case there was any water in the tank. I've driven the car and let it run quite a bit since then with no improvement. I've also noticed that when i hook up my timing light, it doesn't seem to flash consistently. It seems like it has a miss here and there, and will only flash steady when i clip the sensor directly on the top of the distributor cap where the spark plug wire connects. The spark plugs and oxygen sensor were covered in black soot when i pulled them out.

Here's what i have replaced or checked so far:

Fuel filter
Spark plugs
Distrubutor cap and rotor
Checked plug wire resistance, all seem ok (just replaced them 6 months ago)
MAP sensor
Ignition module
Ignition coil
Oxygen sensor
Tested TPS with a multi meter, voltage seemed ok
Cleaned out IAC valve
Spray cleaned intake butterfly into manifold
Checked the Fuel pressure regulator with a vacuum tester, and it holds pressure.
Checked and replaced some vacuum lines, but I'm sure i missed a few.

I haven't checked the fuel pressure yet, since the gauge i loaned from the auto parts store leaked when i tried to use it. I replaced my fuel pump back in the spring, so that hopefully isn't it. Obviously I'm missing something here. What else do you guys suggest?

Thanks in advance,

BiggTipp7

Did you replace the spark plugs or just check them?

Base91

iTrader: (1)

You need to check the injectors too. If they are original they almost certainly need to be replaced. Check the resistance and go from there.

mb2hurley

i have a 3.1 as well. and had alot of those same symptons and drove me crazy, i did everything you did and even got to the injectors just to find out that they were all in range. but after doing all the and still having no improvement, i was doing some of my own research. i ran across something that for some reason never came into my head and i just changed it. i dont know if you can even test them, it was like 40 bucks. i replaced my map sensor. and noticed a huge improvement, im still having a couple other problems. mainly running rich problems but i gotta change my plugs again, they i belive are fouling out from before that, but after changing that part out. my car ran alot better and smoother and got power back. just a thought, something to look into

antares57

iTrader: (8)

Like Base91 said...check the injectors. My V6 3.1 FPR was also bad but the injectors is what was holding it back.. Ohm'd the injectors and found two bad ones. Bought a rebuilt set from SouthBay and fixed my problem. Also check those other vacuum hoses. I replaced all of mine with new hose. Good luck.

thatoneguy5x

what brand spark plugs are you using? this probably isn't your problem but when I did my tune up I put bosch platinum in and it would stumble here and there I was running rich and within a week they were pretty covered in carbon. bought new spark plugs but this time delco and runs 500% better. just a thought. good luck

mikenj73

First of all, thank you all for your help. It ended up being the fuel injectors. Found out that 3 of them were bad. I hadn't even bothered to check them before, because I just replaced all of them just under a year ago. Fortunately, they were still under warranty, so I ordered new ones and sent the old ones back for a refund. Once the new injectors were put in, the car seems to run a lot smoother now. There is still an issue, though. The car will still not rev past 4500 RPMS in Neutral or while driving. It will bog down if you hold it there. I also hear a slight rattling or ticking sound from what seems like the front of the engine. It only happens when it gets to about 2000 RPMS (both in neutral and drive). Anyone know what this could be?
Thanks again for your help!

mikenj73

replaced them with new.

mikenj73

Replaced the MAP sensor, but saw no difference.

Base91

iTrader: (1)

Give Seafoam a try. After replacing the injectors and fuel pressure regulator diaphragm on mine it ran ok but wouldn't rev above about 3k in gear. After a few tank fulls of gas with Seafoam it started to run better every time I drove it and it'll now rev to at least 5.5k (haven't tried to go higher). I figured the years of running rich left a lot of "carbon on the valves".

mikenj73

Yup, I have a bottle of seafoam and high test gas in there now. So far I have only used about a quarter tank since this started happening. Mine will rev up to 4000, but if I try to go higher, it will back down to 4000 again. Hopefully it will get better as I drive it. I may just have to clean the plugs to see if that helps. Not sure if the converter is clogged, or if there is an issue with the fuel pump. I replaced the fuel pump six months ago, so I don't think that's it. I will try a few other things and update you guys as I go. Thanks again for all your help guys I appreciate it.

jakemussman

iTrader: (3)

Does it do this all the time? Or only after the car has warmed up?

mikenj73

All the time. Here's the other thing I forgot to mention: I had to cut the old fuel filter out because the bolts were seized. For now, I have a section of rubber fuel line in its place, but put an in-line filter in there too. The filter is not rated for fuel injection, so I'm hoping that I'm just not getting enough fuel through it. Haven't had the chance to work on it yet due to weather and work schedule, but I will get on it ASAP.

mikenj73

Still haven't had time to go work on the car yet, but I got some advice from my brother in law that it may indeed be my catalytic converter. Once I check, I'll update these notes.

mikenj73

Ok, so it's not the catalytic converter or the fuel filter. I just hollowed out the cat and replaced the fuel filter. I'm just about out of ideas at this point, other than maybe replacing the computer. Anyone else know what it could be?
Thanks in advance,

Base91

iTrader: (1)

It may take several tankfuls of gas with Seafoam to make a real difference. I started noticing a small improvement after a couple of tankfuls but it took several more for it to rev really freely.

jakemussman

iTrader: (3)

Have you checked the coolant temperature sensor?

mikenj73

I'm just worried about that tapping/rattling noise I mentioned before. I don't wanna keep driving it and end up making it worse. Not sure where it's coming from exactly, but it sounds like the front passenger side of the engine somewhere. Only happens after 2000 rpm. It doesn't seem to speed up with the rpms, so I don't think it's valves or anything like that. I'm gonna check the knock sensor today. I read it's in the same area as where the noise is. Hopefully that's it.
You said yours wouldn't rev above 3000 in gear, but what about in neutral? Mine won't go over 4000 in gear or neutral. Sometimes it gets to almost 4500, but then bogs out and drops to 4000. Did you have any ticking or rattling noises under the hood when it acted up?

mikenj73

I haven't yet, but I did get a new sensor. Just haven't put it in yet. Maybe today I'll get to it. Wasn't sure if that could cause that problem or not.

ex-x-fire

Have you checked the fuel pressure yet?
The tapping noise might be signaling your knock sensor to retard the ignition.

Base91

iTrader: (1)

I've got a few stray ticks but a broken exhaust manifold bolt so the exhaust noise from that drowns out other small noises. Before Seafoam in neutral it would get to a little over 4k rpm but not smooth.

mikenj73

I thought the same thing. Just to see if it made any difference, I've disconnected the knock sensor to take it out of the loop. So far, it doesn't seem to do anything different except make my check engine light come on. Someone else I spoke to thinks it's my fuel pump. I just replaced it back in june, so I really hope thats not it. I tried to check the fuel pressure before, but the gauge i got from the auto parts store had a leak in it. I have to get another one and try again.

mikenj73

I'm gonna probably do what you did, and just run a few tanks of gas through it to see if it gets better. If it doesn't, I might have to replace the fuel pump again for the second time in less than a year. Man, I really don't wanna have to drop that tank again. What a pain in the *** that was.....

mikenj73

Just replaced it the other day. No difference.

Maverick H1L

iTrader: (8)

A lot of stock systems have a rev limiter that kicks in around 4500 when in Neutral. The engine speed will go up and down rapidly as the ECM tries to keep the engine from killing itself.

And in gear, our engines are pretty well useless above 4500 due to head and intake design...

Do a compression test. You may have loose valves. I know I have rocker nuts that keep loosening themselves and driving me nuts. Which is why I'm going to switch to non-adjustable rockers. And that may contribute to noise that the knock sensor is picking up.

mikenj73

I'll definitely check the compression and the valves. However, I would think that if there was an issue with the valves, it would show up just as much (or more) at lower RPMS. I'll check anyway, though. The engine has 190k on it now, so I'm on borrowed time as it is. I think the knock sensor is working, but probably wasn't the problem. It doesn't act any differently with it disconnected, but the check engine light comes on, which tells me it is probably still good.

jakemussman

iTrader: (3)

I wouldn't say it is on borrowed time. They are durable little engines. I had over 235k of rough miles on mine and it still ran great. I almost felt bad when I pulled it.

mikenj73

Ok, another update: I noticed that when I slowly press the gas all the way down, it will rev up to at least 5500 RPM. If I rev it quickly, it will hit 4500 RPM and then bog out and drop to 4000 RPM (as it had before). It seems to be an issue of not being able to get enough gas quickly. So, I'm now guessing that it is either a fuel pump issue or possibly a fuel line or fuel vent issue. I still haven't had time to really drive the car, but I will do that gradually over the next week or two. If it ends up being a bad gas issue, at least I will end up saving time not having to drop the tank. I just replaced the fuel pump about 8 months ago, so I'm really trying to avoid doing that again if i don't have to. Anyone have any other ideas?

mikenj73

Yet another update: Not sure if anyone is reading this, but I replaced the computer and there is no difference in the way it revs. I had previously unplugged the knock sensor, and that was not the issue either. However, I did replace a few of the vacuum lines, and it does seem to accelerate a little bit better until it hits the wall at 4000 RPM. My MPGs are back up where they should be. I averaged 25 MPG on my last tank, and got about 310 miles out of it (mostly highway). I'm not sure if its just wishful thinking, but it seems to have improved a little over the last week or two. It still won't rev all the way up, though. Would dirty plugs cause this problem? I'm not sure, since it seems to run smoothly, but just lacks power.
I still have not had a chance to replace the intake manifold gaskets due to bad weather and lack of time. If the gaskets don't do it, it may very well be the fuel pump, which will have to wait until spring. As i mentioned before, I just replaced it about 8 months ago, so I really don't want to do that again if something else fixes it. If anyone else has any ideas, I'd be very willing to hear them.
Thanks again,

Maverick H1L

iTrader: (8)

Check your fuel pressure again. The issue may not be the pump but somewhere between the fuel pump and the rail (a lot of times it's between the pump and the outlet metal line in the gas tank due to rotting pulsator or hose/tube connecting pump to sending unit outlet).

Pontiac400

My brothers 92 firebird had the same problem and we tried all the same things with little to no change and the fuel pressure showed to be normal. On his car it was the diaphragm in the fuel pressure regulator leaking and allowing too much pressure at the injectors basically flooding the engine with gas resulting in black plugs and misfires and a hesitation to rev.

armos

When you check the fuel pressure, make sure you also check it with engine running and when you snap the throttle open. The fuel pressure regulator is referenced against manifold air pressure and should move in relation to it. If it doesn't, check vacuum connections.

If fuel pressure is low, try to find somewhere to pinch off the return line from the regulator. If pressure goes back to normal or higher, then regulator is probably bad.
If pressure is still low, then make sure it's not just the fuel filter. If the car had fuel problems before then it's possible you might have gunk in the fuel lines, which might be putting a strain on the pump and causing repeat failures.

If you have bad vacuum at the MAP sensor, it can also keep the MAP sensor from operating correctly.
You can test the throttle position sensor with a multimeter. 2 of the pins should have a resistance that changes gradually depending on the throttle position. Or you can just backprobe the signal wire as voltage vs ground.