'88 Iroc L98 automatic all stock except K&N filters and flowmaster. i cannot figure this thing out. vacuum rapidly bounces between 8-11 Hg. it smooths out and goes higher with more rpm, and the engine runs really bad, rough with no power. 99% sure that there is not a intake vacuum leak, i just replaced all the manifold gaskets, and the idle is steady at 800ish rpm. the diagnostics sheet for the vacuum gauge says this "Gauge reads below normal and fluctuates rapidly over a range of about 3 in.Hg. at idle, then the needle becomes steady as engine speed is increased" means : Worn intake guides. i removed the keepers and springs, and it seems that the guide is cast into the head itself and doesn't look replaceable, can anyone confirm this? but wouldn't a compression test show if there was something wrong with the guides? i averaged 150 on the compression test with a max of 3% variance, so that checked out ok. i have not done a leak down test yet, as i do not have a tester. i am also not getting any codes. i tried different timing settings, with no change, ignition system is in top shape. also i cannot correctly set the fuel pressure do to the vacuum fluctuating so much, that it is also making the fuel pressure fluctuate between 40-50 constantly, i changed it around a bit, but nothing changed. I'm stuck and don't know what to do, please help.

 

hey ,id try readjusting the iac&tps settings .check out the tech articles on the tgo home page it will tell you how to adjust both correctly.double check for vacuum leaks.doesnt take alot to make one idle funny,later jimmy

 

IAC & TPS won't cause the engine to run rough and have no power. They could be off; but they're not the casue of the problem. Stay focused on things that CAN ACTUALLY produce the observed symptoms, until you solve them.

You've got a fouled plug, burnt plug wire, or defective injector; or something worse. Although, with compression gauge readings within 3%, it's probably something outside the motor.

Check both INJ fuses; check all 8 injectors with a noid light; take off all 8 plug wires and inspect them carefully from end to end; take the whole rails and injector assembly off the motor, and pretend to try to start the car, and see if all 8 squirt fuel equally (DANGER! Watch out for shooting fuel everywhere and starting a fire).

 

i checked both INJ fuses, along with all other fuses, and they are all good. i heard about people doing the noid light thing, i have an autoXray that tells me the injector pulse miliseconds.. isn't that just as good? i also inspected the plug wires, they all seem to be fine, i also checked all the wires with a timing light and flashes on each wire. but i dont think an ignition problem would produce the symptoms im getting, would they? either way the ignition systems is in good shape. no need to check the injectors because i just replaced them with ford injectors that i just got back from rich @ cruizin performance, so im confident that they are in good shape. i bought them hoping to fix the rough running problem i was having, but after installing them, i still have the same problems. i really dont know what else to check, is a leakdown test in order even though the compression test checked out ok?

 

Ah, some HISTORY comes out. That makes a difference. Something that starts up after it's been worked on, is A WHOLE DIFFERENT MATTER from something that just pops up out of the blue for no apparent reason. Was it doing this BEFORE you changed them? I'm guessing, not?Of course not.

All that tells you, is what the ECM thinks it's telling the injectors to do; but NOT the signal that's actually reaching them. Big difference.

Just putting known good injectors in, doesn't guarantee that THEY'RE ACTUALLY DISCHARGING FUEL. There's this whole great big THING all around them, that has all sorts of parts that can cause perfectly good injectors not to do their job; it's called THE CAR. Check all your work from when you changed them out. Don't just blow off the possibility that no fuel is coming out of them. Check it. You'd feel REAL STUPID if you chased this problem for the next 6 months, and it turned out that the problem is that one of the injector bank connectors not plugged all the way in, or something simple like that. INSPECT IT CAREFULLY. Verify that what you THINK is happening, is REALLY happening. No "but... but... but...". Just do it.

Your symptom is also consistent with an ignition problem. Don't just automatically assume that your "ignition system is in good shape" and blow it off. You could be in "maybe it's this, maybe it's that" hell for the next 6 months, if you've got a burned plug wire that you haven't noticed yet. You'd feel real stupid if that happened, I bet. Once you've verified ALL your work from changing out the injectors, INSPECT IT CAREFULLY. No "but... but... but...". Just do it.

With your compression readings that close, a leakdown test shouldn't be necessary. Your problem is ON THE OUTSIDE of the motor.

See my signature for helpful troubleshooting advice.

 

i only replaced the injectors because of the symptoms i was getting, i was hoping that the new injectors would fix it, so the problem has always been there, it didn't develop because of the injector change. i had a 89 iroc in the past that ran like this minus the low erratic vacuum and i replaced the injectors in it, and it ran perfect, so i assumed it was the same problem. but not this time, it still runs the same from before and after the injector change.

im going to look into getting a noid light to test the circuit. but you are saying that my symptoms are consistent with an ignition problem? i think an ignition problem would still pull normal vacuum unless it was a really bad ignition problem. the reason i say the ignition system is in top shape is because i just replaced the plugs, wires, cap, rotor hoping to cure this problem, and it looked like it needed it. the only part of the ignition system that i haven't replaced is the ignition module, i could replace it, but then i am just throwing parts at it, and i think the module is around $40-$50.

autozone does free testing of the ignition module, but i dont know how accurate those tests are, i took my alternator to two different autozones here to get tested, and both stores machines said it had mechanical failure, going with my gut i used it anyway, and that car ran fine and still does a year later, and the alternator still puts out enough amps.

 

You have a "one cylinder doesn't work" type problem.

No an engine with an ignition problem will NOT pull normal vacuum. It will cause the engine to have to draw in more air to avoid stalling, which will lower the vacuum. Furthermore, if ONE cylinder has an ignition problem, then THAT cylinder will cause an internmittent heavy load for the next cyl or 2 in the firing order, which will create low vacuum as they attempt to recover to the correct idle speed. What you see on the vacuum gauge is low, fluctuating, rhythmically pulsating vacuum.

Might be more than one cylinder; but it's not something that affects ALL cylinders EQUALLY.

Therefore, you need to be looking at things that can affect ONE cylinder at a time, or GROUPS of cylinders.

The ignition module is the same for all cylinders. Besides, they tend to fail in specific patterns: works/doesn't, works cold/fails hot, things of that nature. Not likely to be that.

It's not impossible that it's a fuel delivery problem; although, again, not likely, because ALL cyls share the SAME fuel pressure EQUALLY. But it's always worth a check since it only takes like 3 seconds. What's your fuel pressure?

Identify which cylinder it is. Start the engine up cold, and dribble water on each header tube. Won't be hard to figure out which ones are working and which aren't. Concentrate your attention toward ones that aren't.

Use logic and reason, and don't assume that an entire system is good just because you've replaced the easy-to-get-to parts in it.

 

i cannot get a steady fuel pressure either due to the vacuum not being steady, it fluctuates in sequence with the vacuum gauge between 40-50 psi.

it only has manifolds on it, no headers, so i would think that the water trick would only work if it showed it on the front cylinders, right?

btw... thanks for all your help through this sofa.

 

Of course it does. You can always disconnect the vacuum line to the FPR to check the FP. But if it's in the 45ish neighborhood, it's not the problem anyway. You'd be looking for something down below 35, as a potential culprit. If it's over 40, it's not the problem. It's eliminated. Move on.

The water trick works fine if you catch it when it's cold, and dribble the water on during the first minute or so of operation.

You gotta TRY it dude; instead of thinking up reasons for why something WON'T work, get out there, and find things that DO work. All you need to do at this point, is figure out which cyls aren't working, and zero in on the cause. This is just a car, it's not brain surgery or the secrets of the universe or something, it's EASY if you just put your MIND and your WILL (instead of your "can't") to it.

 

From what you describe on your vaccume gauge, it sounds to like a valve problem, In my dealings over the years normally when you see a needle on your gauge brounce around really fast like that its usealy a valve issue! You really should perform a cylinder leakdown test, and all readings should be roughly no more than 30 to 40% depending age and engine milage, of each other as far as the amount of leakdown goes. HOPE THIS HELPS!!!!

 

i dont know what i was thinking about the fuel pressure, i knew i was supposed to unhook the vacuum line to check it, well i rechecked it, and i got it steady at 53.

i tried the water on the manifolds trick, and the water seems to evaporate equally on the whole manifold. after warming it up, i took my temperature gun with a laser pointer and ran it along the manifolds and the temperature was pretty consistent along the whole manifold.

while i was running the engine and reving it up a bit, i heard this noise and felt it on my hand while holding the throttle open, it sounded like someone threw a bolt at a piece of metal, but came from inside the motor, it only did it once, and it did not effect anything, so i dont know about that.

i really wish i could get a hold of a noid light, nobody has them around here, one guy told me that he used a voltmeter to test his injector clips, but if that would work, it seems like there would be no need for a noid light to exist.

i understand that its not something that affects all cylinders, but does affect one or a group of cylinders. what can affect only ONE or a GROUP of cylinders. only thing i could think of that operates individually is the valve train, i believe it is more than one cylinder that is suffering because i've seen a V8 only running on 7 cylinders, and its not this dramatic

 

No. The right thing to do isSame with the infrared pyrometer. If you let it run for awhile, the whole chunk of cast-iron will equalize its temp. You want to catch it while the non-working ones are still cold.

Besides valve train and compression problems (which are largely ruled out if all cyls are between 145 and 155 or thereabouts on your gauge), things that can affect ONE cylinder or a GROUP of cylinders are, the fuel deivery system and the ignition system.

 

i did the water test prior to the engine warming up, i put the infrared meter on after a minute of run time to compare each bank, to see if it could be affecting only one side of the engine. as for the water test, each cylinder seemed to burn off in the same amount of time. as for the compression test, i was wondering if maybe some oil is actually sealing the rings up giving a false reading.

 

i had a similar problem on a car had checked everything and couldnt figure out what was wrong and one day I looked at my tail pipes and noticed I had water cominig out so i checked the radiator and noticed that my hoses didnt have any pressure and when i took off the radiator cap my water was bubbling turned out to be a head gasket replaced gaskets vaccum went up car ran perfect might check this and c if it helps you

 

when you tuned up the car to correct the stumble it is possible that you got 5 and 7 backwards on the cap. try reversing their positions and see if the car runs better. something to check and rule out. it happens all the time

 

i rechecked the wires 3-4 times to make sure they are in the correct spot, i know they are in the correct spot.

as for a head gasket... a compression test would show that.

im going to have to check a few fuel components in the next couple of day, and see where that leaves me.

 

General vacuum guage diagnostics:

1. Steady 17-22 in Hg indicates normal engine in good condition.
2. Low (15-20 in Hg) but steady indicates late ignition or valve timing, low compression, stuck throttle valve, leaking carburetor or manifold gasket.
3. Steady but dropping regularly indicates burnt valve or improper valve clearance.
4. Dropping gradually at idle indicates choked muffler or obstruction in exhaust.
5. Slowly dropping to zero as engine speeds up indicates choked muffler.
6. Fluctuating between 15 and 20 in Hg at idle indicates stuck valve or ignition miss.
7. Drifting indicates improper carburetor adjustment or minor intake leak at carburetor or manifold.
8. Fluctuating as engine speed increases indicates weak valve springs, worn valve stem guides.
9. Vibrating excessively at idle but steady as engine speeds up indicates worn valve guides.
10. Vibrating excessively at all speeds indicates leaky cylinder head gasket.

Testing at various speeds
engine starting vacuum should be 1 to 4 in Hg.
To test disable ignition (ground wire from coil), hold throttle
wide open, crank engine slowly with starting motor.

• Healthy engine at idle should read steady 15 to 20 in Hg.
• Healthy engine at 2000 rpm should read steady 19 to 21 in Hg.
• Healthy engine at open throttle should read close to 0 in Hg.
• Healthy decelerating engine reading should jump to 21 to 27 in Hg as open throttle released.

Testing at idle speed

1. Low and steady usually indicates leaking gasket between intake manifold and carburetor or throttle body, leaky vacuum hose, or incorrect camshaft timing.
2. Low fluctuating (3 to 8 in Hg below normal) reading may indicate intake manifold gasket leak at an intake port or faulty injectors on port-injected engines.
3. Regular drops (2 to 4 in Hg) at a steady rate indicate probable leaking valves.
4. Irregular drops indicate possible sticking valve or ignition misfire.
5. Rapid vibration (4 in Hg combined with exhaust smoke indicates worn valve guides.
6. slight fluctuation (1 in Hg) indicates possible ignition problems.
7. large fluctuation (10 in Hg) indicates likely weak or dead cylinder or blown head gasket.
8. slow movement through wide range indicate possible clogged PCV system, incorrect idle fuel mixture, or gasket leak between carburetor, throttle body, or intake manifold.

Testing at higher speeds

• Rpid vibration (4 in Hg) in reading at increased engine speed indicates leaking intake manifold gasket or head
• gasket, weak valve springs, burned valves, or ignition misfire.
• reading returns slowly to normal and didn't peak above normal (5 in Hg) after dropping to zero when throttle quickly snapped open (2500 rpm) suspect worn rings.
• Rading returns to normal after long delay when throttle quickly snapped open (2500 rpm) suspect blocked exhaust.

Testing for blocked exhaust
Ile speed reading slowly dropping toward zero indicates exhaust restriction.
excessive backpressure in exhaust then indicated by reading not increasing quickly to about 16 in Hg when engine speed slowly increased to 2000 rpm.
backpressure also indicated by reading not dropping as quickly when throttle quickly released and remaining 5 in g higher or more than normal.
disconnect exhaust manifold from engine and retest. If problem disappears exhaust system is blocked.