Well I've been searching this problem and found several problems that were similar to the one I have. I have checked the battery, alternator and harness but this would not explain the voltage jumping only when hot. The ABS light will come on when the voltage hits a low and I apply the brakes. I'm assuming that this is only due to low voltage although I could also have an ABS problem.

Basically, the car turns over without much power and barely gets itself started. Once started, the car has a rough idle. When the car warms up, the voltage will jump around when at low RPMs and the mph gauge will even jump up to 90mph+ when I'm going around 20mph. I have recently replaced the ICM and have tried running in open loop (no change). I plan on getting the fuel pressure tested sometime this weekend and perhaps checking some other common factors such as the coil and IAC/TB.

Any advice on this matter would be greatly appreciated. Thanks.

EDIT:

Also I would like to add that the opti was replaced a year ago so I'm hoping that it isn't the culprit.

 

Try sticking a couple of washers behind the icm/coil. Lots of people do it on the board to help on cooling the module.

Z

 

that does work very well most times, but i dont think its going to help him. i think he has an electrical problem somewhere, considering the speedo jumps to 90mph for no reason.

EDIT: Now that I think about it, the ICM cooling mod might help. The voltage is probably jumping up and down due to the rough idle. That might explain the wierd gauge things goin on.

 

Well after work I took off the intake completely, cleaned it out as well as unplugged and reconnected a few key ignition components such as the coil, ICM, coil wire. When I started it back up it seemed to idle a lot better with the intake off. It will still drop to low voltage but then I would hear a tick and it would pick the idle back up to 12v out of the red zone, then slowly go back down, then tick, etc.

Any further suggestions would be appreciated, thanks.

 

check to c if teh alternator is working properly ,check to c if its giving teh right voltage etc i had a rough idle with my beretta, the rpms willjump around as does the voltage, new alternator fixed it,

electrical problems can cause some weird stuff

 

Check and clean all of your grounds. Make sure you don't have any melted or cut electrical wires. Clean grounds never hurt.

Z

 

My bet is a real bad misfire. I have observed the Alternator voltage with the OEM and a brand new replancement... when the RPMs fall at a certain speed the alternator will drop the voltage. I did it with my original OEM and with the brand new replacement at Idle... I would make my lights flicker at will.

Since you hear a click and then the voltage drops faster than a Pelican on a school of Anchovies I get the feeling it has to do with the misfire.


My background knowledge on this problem based on the Time I went all freak with the new Lifters Adjustment.... I adjusted them too tight and when the engine got past 200 degrees it began to misfire like snoop doggie dog when the lakers loose a game At that point my dash voltage was all over the board.... up and down.. As soon as I re-adjusted the lifters and crancked her back up... the problem got fixed.


Check for Sparks in a peach black night. I am almost positive you have a bad misfire somewhere.


Marvin

 

phibud,

I have the exact same problem in my 383. Loss of power, and idle will drop at stop lights, the ABS light will come on, and my voltage will fluctuate.

Any luck figuring out your problem?

 

i had this happen alot, hell still do, tighten your battery cables because sometimes they dont touch properly thats my guess. i know what you mean when it gets warmed up at a light the voltage goes up and down and the lights flicker and dim, tighten the battery cables and see what that does for you.

 

That sounds exactly like what I just went through. My problem turned out to be a bad battery. I was driving and the gauges went all over the place like crazy and the abs light and brake light came on. The car also started running really bad. Haven't had a problem since I put a new battery in. The one that was in there was some brand I had never even heard of.

Hope that helps,
Shawn

 

I have a brand new Die Hard Gold in there, but now that I think about it the negative terminal is stripped a bit, causing me not to tighten all the way... I will try another battery.

I have already changed out all 4 coils on my Delteq which helped a lot, I think one coil was lame (yellowish spark instead of blue from the other three). I checked and rechecked everything ignition-related. Brand new 10.4mm Taylor plug wires, and Autolite 104s gapped @ 0.40.

As far as I know there is no vacuum leak, and there are no holes in my intake elbow, but there is a minor rip before the MAF. I even swapped out my Holly 58mm TB with my friend's BBK 58mm and it did nothing.

Another possibility is my Walbro 255 fuel pump. Last weekend coming to a stop light after a quick run on the highway, I noticed my fuel pump making a loud hissing noise. It has under 1K miles on it, but it may be starting to go. I can't go past about 4,500 RPM without having it stumble real bad so I think it may be the pump. In neutral, I can rev all the way to redline without any hickups. Fuel pressure is good at 43.5 psi with the vacuum line off. Driving today I made the same pass on the highway and the fuel pump is real quiet now... Hmmmm...

One last possibility is my ATI Superdamper which is 6.75" diameter and 10% underdriven. I'm considering trying the 7.5" version, but not sure, will it fit the same aluminum hub? Plus it's like $300+

Though the stipped battery terminal sounds like it could cause. I will let y'all know.

 

I replaced the battery under warranty -- no luck. I have just yesterday finally got rid of all exhaust leaks. Now, the car drives, but after it gets warmed up 5-10 mins, it feels like I lost about 200 HP. When I jump on it it's fast, but not where it should be, and there is a serious stumble after 4K in all gears. The weird thing is that in neutral there is no stumble, and the RPMs are very smooth all the way up to redline, it's only under load that it craps out.

I replaced my fuel pump because it was getting noisy, no luck either. I checked my fuel pressure by taping the gauge to my windshield, and it's constant around 28-30 PSI, no matter if at idle or at WOT, so it's not my regulator. I have the Delteq DIS with brand new coils, so I don't think it's causing this.

The only thing left is my cat. I have a high-flow under there, but Lou's Custom Exhaust for some reason welded without using flanges?? I am planning to go back soon to have them weld in flanges, then I will gut the cat and be done with it. I have never seen it glow red, but my engine is very hot to the touch (VERY) while my coolant temp is under 180*.

Also, I swapped out my TB with a friend, as well as the MAF. Brand new MAP.

Anyone else have any other ideas?

 

Hey Black, Sorry to hear man. It's hard to reply to any online posting based on an online description for many reasons but what I can say is... now that you replaced the battery you know the voltage upto 12 volts will remain consistent. Provided the battery is being charged or was charged properly.

From there ... how bad is the voltage fluctuation? between 12 and 14 volts? or does it drop below 12 volts at ...say... the voltate around the power feed to your ingition?

I would firts compare voltate at the battery and the voltate at the fuse box inside the engine compartment... if the the voltages fluctuate equally but not really dropping bellow say 11volts...(12 with a charged battery) then I say its the result of the engine misfiring causing the RPMs to fluctuate rapidly.


I noticed on my 95 LT1... when I suddenly let off the gas and the RPMs drop rapidly... right before Idle speed my front lights will flicker once and then go back to normal. I discovered the voltage will drop to 12 volts for a short moment and then back up to 13.8. When I messed up with my lifters and made the car misfire..(lifters too tight) the RPMs were fluctuating rapidly up and down and the lights would flicker along with it. Once I fixed the Lifters the voltage fluctuation was gone except as above explained.

So what I did...as a test... (over a year ago) I connected 3 of the 3000mfd capacitors in paralell and hooked them up the the battery positive and negative..... I have not seen the lights flicker ever since... I think I fooled the pcm to think there wasnt any rapid RPM changes. The Capacitors are still hooked up and had not problems at all. Mind you if the car misfires I bet the voltage will still flicker .. capacitors connected or not.

My assumption is the PCM disconects the Alternator if it senses a certain RPM speed Drop. There is a small wired connected to the PCM from the alternator and I think thats how the alternator is switched on/off.

So... If your car misfired badly the rapid change of RPMs is bound to flicker your power......


Of course.. there could be a bad electrical conection somewhere... aka bad ground.... it could be a bad positive (hot wire) contact too. That's why I say compare your voltage fluctuation at the battery .. if its the same fluctuation with everywhere else in the car... I would go back to the alternator... and the fluctiation of the RPMs. If not.. there is a bad contact somewhere.

I heard a spark plug arching does the same effect... I dont know if its due to the overload or the change in RPMs.


Last.... At a few times my car would idle fine... drive fine with about 1/8 of gas pedal... as soon as I press the gas a bit more it would start misfiring..

It turned out to be bad spark plug resistors. My good spark plugs had a resistance of 3 to 4 kilo Ohms... the bad Ones were at 40 to 50 kilo Ohms. The color of the bad ones firing tip was dark brown with lots and lots of dry carbon deposites on and around but not quiet at the tip. The good sparkplugs had a nice light brown tips.


I was using NGK TR5's then TR55's when I had the problems. I switched to Bosh and havent had the problem ever since... over 8 months ago.


If I hadnt found out about the bad resistors in the spark plugs I would have probably chased that problem and eventually, out of stress, set the car on fire...... I hate t0 say it but If it was me I would check each spark plug resistor. Dont rely on brand new sparkplugs to be good.... Remember I was replacing the spark plugs and ending up witht the same problem a week down the line.


I know its a bish to remove the spark plugs.... I have been there several times... but if you already checked everything else, why not. I had a brand new opty and I knew it wasnt the opty.


I heard the when the opty goes... it begins at high rpms.... I changed my opty before it went at 148k miles... so I have never experienced a failed opty... in my case it was alwyas the spark plugs.

Have you changed the fuel filter yet? I once got a cloag one and the car would hit a wall at about 40 to 50 miles per hour or anytime i would press the gas a little bit down.... Changed the filter and went back to normal.

I hope you find your problem man... it could be something simple...as it was in my case...


The tick sound sounds you hear sound soooo much like a wire or spark plug arching.!!!

Marvin

 

Thx for the reply, lots of good info there! Almost everything is brand new since my engine rebuild, including the fuel filter. I know for sure that it's not fuel-system related.

The voltage flux happens a lot less now, and only when I come to a stop light and my RPMs drop below 800 for a few seconds. The flux in voltage is anywhere from 12.5 to 13.5 volts according to the [inaccurate] in-dash gauge. I have not seen it dip below ~12 yet. This RPM loss is NOT due to MAP, IAT, IAC, or TPS. New MAP, tried the resistor mod for the IAT, swapped MAF and TB with friend -- all to no effect.

I have recently changed out my NGK TR55 plugs for AC Delco coppers. Two of the old plugs had high resistance of over 20 ohms, compared to about 5 ohms the others showed. This I found out was linked directly to a failing coil which produced a yellowish spark with plug wires off and cranking the engine. Delteq sent me 4 brand new coils free of charge -- very good customer service IMO.

I have scheduled an appointment for next Wednesday to install flanges so that I can gut my cat. I have been smelling some rotten eggs here and there. Hopefully this is the cause.

 

After over a year of troubleshooting, I have FIXED all my problems!!!

It turned out to be the d@mn opti. I have the Delteq DIS, so I thought the optical sensor would last for a long time, I was dead wrong. When my friend and I pulled the opti off we noticed it rattled alot inside. We took it apart and all the plastic pieces fell out on the floor. The metal disk with the holes in it for the optical sensor looked like it had been mutalated. How this happened I have no clue, but thank God my friend had a spare opti.

About 5 hours later we took it for a spin on the highway -- oh man is this b!tch fast now. Runs absolutely great!! Very happy camper.