Mechanics

HARD STARTS OTHER PROBLEMS

1998 Chevrolet Tahoe • 144,000 miles

I have been having alot of problems with my 1998 Tahoe. I will start by telling you I had my engine rebuilt 2000 miles ago and recently had the distributor moved a tooth to get rid of check engine light (crank/cam code) and replaced a fuel pump 5 years/20,000 miles ago with an ac delco fuel pump. My recent troubles started with a pulsating fuel gauge, then hard starts, then I noticed a faint smell of gas on my oil cap, so I hooked a fuel pressure gauge up to it and it read 50 PSI with the engine on but not started which is low and with the engine started the gauge began to pulsate (moving rapidly) between 40-45 PSI. I also had my battery tested and it tested at 12.47 volts but only 382 CCA and it is supposed to be 700 CCA. Can a bad battery make me have all these symptoms? Bad battery making my fuel gauge pulsate in the car? Bad battery making it hard starts and the hard starts making my oil cap smell like fuel? Bad battery not giving the fuel pump enough voltage throwing off my fuel pressure gauge? Or is it something more like a fuel pressure regulator or leaky injector or even another failing fuel pump?
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Aarnold1
July 21, 2011.




I MAY NOT BE YOUR "FIX IT MAN"

OTHERS STILL MAY JUMP IN AND HELP. AND ARE WELCOME!

I DEAL WITH OLD JEEPS AND FORDS

COMPUTER STUFF IS NOT MY THING. BUT I DO KNOW SOME STUFF!

JUST TO GET THINGS STARTED. SOMEONE WILL WANT TO KNOW MORE INFO

DOES YOUR CHECK ENGINE REVEAL, IT DOES WORK, AT START-UP? IS IT ON NOW? ARE THERE ANY CODES? YOU CAN GET CODED FOR FREE AT MANY POPULAR PARTS STORES!

LET'S START WITH "GAS SMELL". DOES YOUR DIPSTICK SHOW "MORE OIL" THAN WAS INSTALLED. DOES THE DIPSTICK SMELL LIKE GAS

WHEN YOU HAD THE BATTERY TESTED, DID YOU TEST THE ALTERNATOR? AGAIN, FREE AT A PARTS STORE!

IF NEITHER THE BATTERY OR ALTERNATOR FUNCTION PROPERLY, AND ARE NOT WITHIN SPECS. THIS IS THE PLACE THAT I WOULD START. GOTTA BE FIXED ANYWAY! MAY SOLVE YOUR PROBLEM

COMPUTERS ARE SENSITIVE TO VOLTAGE FLUCTUATIONS

RETURN WITH GOOD NEWS

OR

I WILL FIND YOU ANOTHER EXPERIENCED EXPERT TO CONTINUE HELPING YOU.I HAVE A FEW IN MIND!

THE MEDIC

CJ MEDEVAC
Jul 21, 2011.
I currently do not have a check engine light on, when my battery was tested they did test starter and alternator but they said they weren't sure if either was good. Starter was at 10.5 volts and was supposed to be at 12 but they told me it could be a little lower due to the battery? I know for a fact I should replace the battery because I have owned the tahoe for 8 years and have never changed it and who knows when it was changed before I even owned it? So I am wondering could all these problems stem from a bad battery? Or is it a combination of battery and fuel pressure regulator or even battery, fuel pressure regulator and fuel pump? I can hear my fuel pump running when I turn my key over and it runs for just a few seconds to build up pressure and shuts off everything seems normal and its fairly new (5 years/20,000 miles) so I am really hoping that isn't what it is

Tiny
Aarnold1
Jul 21, 2011.
And in response to the dipstick, my oil level hasn't raised as of now but I can faintly smell it on the dipstick but the smell is a stronger on the oil cap. I changed my oil like 3 weeks ago and it is still at the same level.
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Tiny
Aarnold1
Jul 21, 2011.
THE STARTER TESTING RESULTS REALLY MAKE NO SENSE AT ALL (TO ME, ANYWAY). UNLESS MAYBE THEY WERE TALKING ABOUT THE AMPERAGE IT WAS DRAWING?

STILL, THE BATTERY IS NOT WITHIN SPECS.I WOULD ELIMINATE IT FROM THE EQUATION. EVEN IF YOU BORROWED ONE FROM ANOTHER RIG, THAT WAS UP TO SNUFF, JUST TO SEE IF THERE IS A CHANGE.

IN SMALLER TERMS, MY FLASHLIGHT DOES COME ON, BUT IT FLICKERS, I USE IT OFTEN, I PUT THE BATTERIES IN IT 3 YEARS AGO. LET'S TEAR IT ALL APART, AND SEE IF WE CAN FIND A PROBLEM!

THIS TIME SEEK ANOTHER PLACE TO GET TESTED. EITHER THE ALTERNATOR IS DOING IT'S JOB. OR IT AIN'T. THIS IS A "NEED TO KNOW" THING!

I'M NOT SAYIN' YOUR SNIFFER IS OFF, SMELL ANOTHER VEHICLES CAP, IS IT DIFFERENT. THERE ARE EXHAUST GASSES PRESENT IN THE CRANKCASE, IT COULD JUST BE FROM RUNNING RICH. IF THE LEVEL HASN'T CHANGED, AND THE OIL DOES NOT SMELL OF STRONG GAS, WORRY LESS. SORTA A QUESTION FOR ANOTHER EXPERT, IF ONE COMES ON BOARD OR IF I GO GET ONE.

THE MEDIC

CJ MEDEVAC
Jul 21, 2011.
The first thing that concerns me is the fuel smell on the oil cap. Is the compression low even though it was reciently rebuilt? As far as the fuel pressure while running, for a faulty fuel pressure regulator or a plugged fuel filter. You can check pressure before the fuel filter to see if does the same thing. If it doesn't the filter could be the issue.

As far as the battery, to start the engine, you need CCA that are equal to the cubic inch of the engine. Therefore, if the battery is good, has good connections, it should start the engine with ease. The larger CCA requirements are the result of all the ooptions. Check to see if the battery voltage increases to aprox 14 volts with the engine running. If it does, the alt is working properly.

Let me know what you find.

Hi guys. Aarnold1, you don't realize it but you're suspecting the battery and that's a very good possibility. There's no way to sugar-coat it, beginning with the '87 model year, GM went from probably the second best generator design to by far the world's worst pile ever. Due to the "switch-mode" voltage regulator, it turns the field winding on and off about 400 times per second. Average voltage is controlled by varying the "duty cycle"; that's the on-time vs. Off-time percentage. When it turns off, being a coil of wire, the field winding produces a huge voltage spike when the magnetic field collapses, just like an ignition coil does. This is such a problem that they even use special "zener" diodes to short out those spikes.

The rest of the job of dampening and absorbing those spikes is done by the battery. As they age and the lead flakes off, their "internal resistance" goes up and they lose their ability to dampen those spikes. The spikes wreak havoc with computer sensor signals so computers do weird things, and those spikes can easily destroy the internal voltage regulator and diodes. It is REAL common to have repeat generator failures. Many people go through four to six in the life of the vehicle. What many professionals are finding out is to prevent those repeat failures, replace the perfectly good battery at the same time the generator is replaced. That old battery might work okay in an '86 or older model.

I can't say all the symptoms, (nice job of documenting them, by the way), are related to the battery, but especially since it is that old, get a new one in there, then continue with the troubleshooting. At least we can eliminate that potential variable. Everything you mentioned is in some way connected to a computer on the truck so multiple symptoms can be expected. If the battery does solve some of the problems, you would have been chasing your tail trying to figure out some other cause that wasn't there.

As for the starter voltage, the industry standard, unless the manufacturer specifies differently, is 9.6 volts or higher during cranking. This is where that internal resistance comes into play. A good, fully charged battery will measure close to 12.6 volts. Where did that three volts go during cranking? Whenever current flows through anything with resistance there is a voltage drop, just like there's a pressure drop at a kink in a garden hose or compressed air line, but only when water or air are flowing. Battery cables have a little resistance which is undesirable, but that's why a tiny amount of voltage is dropped across them during cranking. We would like ALL the battery's voltage to be dropped across the starter, but a good chunk of it is dropped right across the plates in the battery, ... Two or three volts worth. We can't measure that resistance with an ohm meter, but we can see the results of it during cranking or a load test. Current flow through the battery's resistance creates that two or three volts voltage drop, and you find, in your case, 10.5 volts where you expected to find 12 volts. 10.5 volts is very respectable and means the battery can supply enough current for the demands of the starter motor.

There's two ways to test a battery, and I'm guessing your mechanic either didn't fully understand what he was doing with that piece of equipment or he wasn't good at describing what he did. On higher-class testers you dial in half of the battery's cold cranking amp rating, then watch to see if the voltage stays above 9.6 volts for 15 seconds. If it drops below 9.6 volts in 15 seconds or less, the battery doesn't pass the test, but you have to make a judgement call on how bad it failed by. If it hit 9.5 volts right at the end of the 15 seconds, yeah, technically it failed, but it's still doing a fairly good job if it's pretty old. A one or two-year-old battery that just squeaks by isn't going to get better with age, just worse, so that would be more cause for concern.

Some less-expensive battery testers have just one resistor to draw the same load on any size battery, then you have to look at a chart to see how much voltage it should be able to maintain with that load. A tiny riding lawn mower battery will have the same load put on it as a huge diesel truck battery. The little battery might pass if it can struggle to maintain 6.0 volts while the big battery might fail if it can't maintain 11.5 volts. That's like comparing the weight lifting ability of two men. You would be impressed if a 90 pound weakling could lift 100 pounds, but if the 300 pound giant couldn't even lift 200 pounds, you'd be disappointed. I never used that kind of tester so the voltages I used in my story are just examples. Same with the weight lifters!

The bottom line is your battery passed a load test with flying colors, ... I think. They said they found 12.47 volts but only 382 cold cranking amps. That sounds like the first type of tester I described. They have a variable resistor with a big **** that lets you dial in one half of the 700 amp cca rating. It's impossible to dial it in exactly but 382 is more than the desired load of 350 amps and it still maintained at least 9.6 volts. Assuming they did the test correctly and for the full 15 seconds, cranking-wise, the battery is fine. It still can have high internal resistance though which will allow the generator's voltage spikes to be an issue.

To my knowledge, only GM has the voltage spike problem. Their older "SI" generators from the early '70s through '86 also had an internal voltage regulator and it also switched on and off 400 times per second but they never had this problem. I don't have an answer for that. Chrysler switches theirs the same way but the regulator is in the Engine Computer where it can adjust the target voltage for many other factors such as wide-open-throttle, hot or cold ambient air temperatures, and things like that. They also use a switching regulator without any problems. VCR power supplies, which I'm very familiar with, use the same system. If you understand electrical theory, volts times amps equals power, (watts). Watts equals heat, the deadly enemy of electronic components. When the switching transistor is turned off, 0 amps flows, so there is 0 watts dissipated and no heat. When it is turned on and current flows, there is no voltage dropped across the transistor, so again, no watts, no heat. That means they can control the rate of charge with a tiny inexpensive transistor. Think of letting the water drain out of your bathtub. It takes no effort on your part to let 'er flow as fast as it wants to, and it takes very little effort to totally block the drain with your hand. Full on or full off, with very little effort. If you wanted to slow the rate of draining, it would again take very little effort to let 'er rip for a few seconds, then completely block it for a few seconds, and keep switching that way. If you tried to block exactly half of the drain opening, you would struggle and use a lot of effort to hold your hand in the right spot. THAT'S where a regular transistor used to control the average generator field current would have some voltage across it and some current through it. Multiply those two numbers and you get some watts which equals lots of heat, a big power transistor, and it's mounted to a huge aluminum heat sink to keep it cool.

Way more technical than you need to worry about, but in spite of those voltage spikes, they did have a reason for designing the voltage regulator the way they did. I'm not an engine performance specialist but I can share that the oxygen sensors on your truck develop a small voltage that should switch between around 0.2 and 0.8 volts a couple of times per second. When the generator develops a voltage spike, the higher voltage, (electrical pressure), causes a corresponding spike in current flow to the battery. Current flowing in a wire sets up a magnetic field around that wire. (That's how kids make electromagnets to pick up paper clips with a small battery). The opposite is also true. When another wire is nearby, that magnetic field from the adjacent wire "induces" a voltage in the second wire. That's the way the secondary winding in an ignition coil works. Same for any power transformer in your house. If a big voltage spike occurs in the wire going from the generator to the battery, it can easily induce a voltage spike of a few volts in an adjacent wire in that same harness, and that wire could be for one of the oxygen sensors. The Engine Computer is purposely turning the injectors on and off in such percentages to make the mixture switch rapidly from too rich to too lean, and it expects to see corresponding readings from the oxygen sensors. Now add in these extra induced voltage signals and the computer sees different signals that it may try to correct by making changes in the amount of fuel it commands from the injectors. The result could be too much fuel and poor fuel mileage, or not enough fuel and hesitation and stumbling. Driveabilty experts get involved at that point. A simple trick to identify that cause of a running problem is to unplug the small electrical connector on the side of the generator so it produces no output and no voltage spikes. If the running problem clears up, it's a good bet the generator / battery is at fault.

Caradiodoc
Jul 21, 2011.
I have not had the compression tested yet, I wanted to check battery and fuel pressure before compression as I just had it rebuilt and don't want to believe it already has low compression, but I did read if your fuel pressure regulator is leaking it will create the smell of gas in the oil. So since my batteries volts were within spec but the CCA were really low does this mean my battery is still good? They told me I needed to replace the battery because the CCA's were too low? I need to replace it anyways since its 8+ years old but if it isn't whats causing the hard starts I wanted to get that fixed first. Basically I don't want to spend 100-150 on a battery then turn around have the same problem and dump 500 on a fuel pressure regulator or whatever the fuel issue is.

Tiny
Aarnold1
Jul 21, 2011.
I typed for an hour explaining that your "CCAs" are not low. Something was done incorrectly during the test or it was explained incorrectly. I also suggested it is very possible all the symptoms could potentially be caused by the battery even though it tests okay.

Caradiodoc
Jul 21, 2011.
Does anyone know if there is a vacuum hose that goes into the fuel pressure regulator on this truck? And if so do you know exactly where its located and is it easily accessible? I can't find it anywhere, I was told if I can get the hose off and smell it and if it smells like fuel my pressure regulator was bad? By the way caradiodoc thanks for the informative answer I guess I just wasn't understanding it because when I got my battery tested they said the voltage looked good but CCA's didn't and the print out sheet said replace battery? Thanks in advance for anymore info.

Tiny
Aarnold1
Jul 21, 2011.
It should be attached to the fuel pressure regulator itself. The regulator is on the fuel rail. Also, that is correct. If fuel is getting into the vacuum hose to the regulator, it needs replaced. If you still don't find it, let me know and I'll try to find pictures.


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