Hesitation/Stumble on Acceleration

[mc_marto]

Hi there,

I'm hoping someone would be able to help me figure this out. I have a '91 Deville, which recently started having a hesitation when accelerating. It happens in the 1300 - 1400 RPM range. Sometimes it is harsher, sometimes - softer. It happens when the engine reaches normal operating temperature and closed loop. The engine doesn't lack power however, and the bucking happens only at slow to moderate throttle opening. If you accelerate more rapidly or at WOT the engine performs very well and there is no hesitation. Seems like it's only happening in the RPM range I already mentioned, and since on a faster acceleration you would skip this range - there is no hesitation.

So far I have adjusted my TPS according to my FSM,

cleaned the throttle body, EGR tubes and valve, MAT, MAP (and I have a spare one that I have been using for comparisons), repaired some vacuum connections, changed the Hall Effect Sensor, and installed new cap, rotor & wires.

I am suspecting a failing ICM. I believe this is one of the symptoms, although I don't have hard starting or stalling issues. I know they usually fail from heat, and just coincidently my hesitation comes up when the engine warms up.

I would highly appreciate some help!

[KHE]

ICMs usually fail such that there is a no-spark condition when warm and once they cool down, they will begin to operate again. I would not suspect the ign. control module at this point.

Have you checked to see if there are any diagnostic codes stored? With the ignition in the run position, simultaneously press and hole the OFF and WARMER buttons on theclimate control panel. The codes will be displayed on the fuel data center (I believe for that year car). Write them down and post them back here. When you see .7.0 on th edisplay, the system has completed the code display and is awaiting further input. You can turn off the key or push AUTO to exit diagnostics.

[mc_marto]

I have checked for codes many times already, and nothing comes up. I'm suspecting since the issue is intermittent, the system does not find an issue.

I was reading the symptoms of a failing ICM here http://www.fixya.com/support/r7995890-diagnose_engine_surges_stalls, which states that:

Ignition Control Module failure symptoms

1. High RPM misfire.
2. The car will start and run fine, but will stop running anywhere from seconds to minutes later. The engine may not restart immediately but if left to cool for about 10 minutes will start fine.
3. Hesitation/misfire under acceleration.
4. Failure to start.

My condition started a couple of weeks ago. I was hearing my distributor making a kind of grinding noise, so I suspected something might be wrong with the gear on the distr. shaft. I took the distributor out, and inspected the gear which turned out to be fine. There were a couple of plastic pieces in the hole, which I don't know where they came from. Anyway, I fished out those pieces, and noticed some corrosion on the hall effect switch so I sprayed it with an electronic cleaner and scrubbed with some steel wool. Well, since there's a magnet on the hall effect and pickup coil there were a lot of metal shavings from the steel wool. While attempting to remove them with a screw driver I broke the hall effect sensor. I re-soldered the leads and glued the magnet back. Put the distributor back in aligning it with my markings.

Well, even though I had it aligned with my markings my timing was still off. This is when I noticed my condition present - when the car warms up there were two noticeable hesitations in 1st and one in 2nd gear. Since I did not have a timing light at the time and it was a late evening I had to wait till the morning to get it timed. I went to a friend of mine the next day, around noon, and while driving the car cold at first I experienced no issues, so I drove around the block before I got to my friend's house. Well, as soon as the car warmed up the hesitations started again. When I got to my friend's place I started the car for a bit and the car was idling very rough as well. He did not have a timing light so I went ahead and bought one.

I timed it the same day, but my hesitation is still there most of the times after the car warms up. It's certainly not as big of a hesitation as it was prior to timing, but I can feel and hear it. I have observed the RPMs on P.0.8 and it happens only in the 1300 - 1400 RPMs, and there is a drop of 50-100 RPMs when it happens, and it resumes up quickly after that. I suspected my hall effect switch bad, so I performed the test described in the FSM and the test proved it defective, so I got one off a '94 Deville from the Pick'n'Pull and it's still the same. Yesterday I tested the "new" hall effect and it is working correctly.

Changing the cap, rotor & wires did not resolve anything either.

P.S. I'm sorry for the lengthy post, but I think every bit of information could be a clue. Thank you in advance for looking into my issue!

[KHE]

Not sure how you heard a grinding noise from the distributor. Was the noise present after turning off the engine? It sounds like it may be the idle speed control motor setting up for the next start. If the closed throttle switch is not making proper contact, the system will make a grinding noise as it attempts to ready for the next start. There is a ton of information on the ISC in the archives.

[Cadillac Jim]

After you get done following KHE's excellent advice, I have some different thoughts. But make sure that the ISC is good first.

If you busted the distributor and can't get the timing right, you hear noises from it, and have driveability problems attributable to ignition, you should probably get another distributor from a recycling yard.

The modules can detect faults in microseconds. An intermittent fault for something that is under constant monitoring that throws a code *will* throw a code, no matter how brief. Some things, like some network connections, aren't under constant monitoring.

The symptoms you describe sound a lot like a coil breaking down. That won't throw a code.

[mc_marto]

I know it's normal for the ISC to be ratcheting back and fourth after you shut the engine. The noise I described is present when the engine is running, and it's coming from the distributor. Maybe I'm hearing the coil firing and the spark jumping to the terminals inside the cap. It's not a loud noise, though, so it's probably normal.

So, you say the ignition coil is probably going bad? Can I perform some testing on it?

[Cadillac Jim]

I think that the symptoms you describe can be caused by a coil breaking down, but without looking at the car I can't say that it is *probably* the cause. I just did a check on Rock Auto and your coil is an HEI coil. Check the primary and secondary resistance, and check for shorts to ground and across the coils. Usually when a dry ignition coil goes bad, it is arcing inside and the arcs generate conducting tracks that can be found with an ohmmeter.

[KHE]

I know it's normal for the ISC to be ratcheting back and fourth after you shut the engine. The noise I described is present when the engine is running, and it's coming from the distributor. Maybe I'm hearing the coil firing and the spark jumping to the terminals inside the cap. It's not a loud noise, though, so it's probably normal.

So, you say the ignition coil is probably going bad? Can I perform some testing on it?

The ISC should not be ratcheting when you shut off the engine. It should move the throttle plate to the proper position for the next start. It should not be ratcheting back and forth.

There is a way to check the closed throttle switch while in diagnostic mode - one of the lights on the switches on the climate control panel will indicate the ststus of the closed throttle switch. I do not recall which one but that information should be in the service manual - someone with a 1991/1992 manual should be able to post it.

There should also be resistance specs for the coil in the shop manual.

[adallak]

A bad ISC motor would not cause hesitation under acceleration. Ignition system is the main suspect. Even bad wires could do that.

[KHE]

A bad ISC motor would not cause hesitation under acceleration. Ignition system is the main suspect. Even bad wires could do that.

Right - I was just pointing out that the ISC should not ratchet back & forth.

[KHE]

Hi there,

I'm hoping someone would be able to help me figure this out. I have a '91 Deville, which recently started having a hesitation when accelerating. It happens in the 1300 - 1400 RPM range. Sometimes it is harsher, sometimes - softer. It happens when the engine reaches normal operating temperature and closed loop. The engine doesn't lack power however, and the bucking happens only at slow to moderate throttle opening. If you accelerate more rapidly or at WOT the engine performs very well and there is no hesitation. Seems like it's only happening in the RPM range I already mentioned, and since on a faster acceleration you would skip this range - there is no hesitation.

So far I have adjusted my TPS according to my FSM,

cleaned the throttle body, EGR tubes and valve, MAT, MAP (and I have a spare one that I have been using for comparisons), repaired some vacuum connections, changed the Hall Effect Sensor, and installed new cap, rotor & wires.

I am suspecting a failing ICM. I believe this is one of the symptoms, although I don't have hard starting or stalling issues. I know they usually fail from heat, and just coincidently my hesitation comes up when the engine warms up.

I would highly appreciate some help!

When was the last time the spark plugs were replaced? If the platinum plug tips come off the ground electrode, the plug gap opens up and it can cause hesitation. I had this on a '96 Seville a few years back.

[mc_marto]

The ISC should not be ratcheting when you shut off the engine. It should move the throttle plate to the proper position for the next start. It should not be ratcheting back and forth.

There is a way to check the closed throttle switch while in diagnostic mode - one of the lights on the switches on the climate control panel will indicate the ststus of the closed throttle switch. I do not recall which one but that information should be in the service manual - someone with a 1991/1992 manual should be able to post it.

There should also be resistance specs for the coil in the shop manual.

I want to clear the confusion. When I said ratcheting back and forth I meant adjusting for next startup - it does a couple of short movements to do that. That's what I meant. The closed throttle switch indicator while in diagnostics is the Off indicator on the display. Everything is fine with that.

I believe I performed a test from the service manual on the coil, but I can't recall the values I got. I will look into that again.

When was the last time the spark plugs were replaced? If the platinum plug tips come off the ground electrode, the plug gap opens up and it can cause hesitation. I had this on a '96 Seville a few years back.

I installed new spark plugs on May 18, 2011. They are Autolite Double Platinum.

I don't know if I mentioned this already, but when I replaced my spark plug wires last weekend I discovered the wire for cylinder 4 was burnt from touching the exhaust manifold. Could it be possible that the burnt wire was causing some misfires in cylinder 4 and as a result of that the plug could be fouled?

I visually inspected the plugs in the front (cyls. 1, 3, 5 & 7) a couple of weeks ago, and they seemed fine, but I did not check the ones on the back - where cylinder 4 is. Should I get into checking those?

[mc_marto]

I tested the ignition coil again. The primary winding reads 0.5 Ohms and the secondary without the carbon brush is 9.25 kOhms (with the brush it is 13.15 kOhms, so the brush is about 4 kOhms). Those numbers seem fine, don't they?

The pickup coil is supposed to be in the 500-1500 Ohm range - it is at 800 Ohms, so it checks out ok as well.

The only thing not tested remains the ICM. I guess I can try to take it to a parts store tomorrow and have it tested.

I will also check the plugs tomorrow, but they should be ok.

[Cadillac Jim]

I don't thing that the carbon brush should be 4K, but I don't know what the carbon brush *is* so I'm at a bit of a disadvantage. Those with manuals or experience with the 4.9 ignition may be more helpful but I think 4K is a red flag.

[adallak]

I would make sure the wires do not touch the exhaust manifold. I would also put some dielectric grease into the boots of spark wires. Can't say much about the resistance readings. I do not have my 1991 Seville FSM anymore.

A weak EGR valve could cause hesitation, but yours looks ignition-related. Replacing EGR valve on a 4.9 engine is such a pain. Actually, what is not on 4.9?

[KevinW]

Is the EGR valve original? It is possible an aftermarket replacement is allowing too much EGR flow, which is not a factor at higher throttle openings. There might also be a PCM calibration update to address EGR chuggle conditions. Does it idle smoothly? It might be helpful to measure the injector coil resistances (while hot) and perform an injector balance test; in Diagnostic Mode, individual injectors can be disabled and the resulting RPM drops can be observed.

[mc_marto]

I don't thing that the carbon brush should be 4K

You know, I thought about it being weird too. I'll check tomorrow what the resistance of the brush in my old cap is.

A weak EGR valve could cause hesitation, but yours looks ignition-related. Replacing EGR valve on a 4.9 engine is such a pain. Actually, what is not on 4.9?

I replaced the EGR valve last summer. My old one had gotten a stiffer diaphragm spring that wasn't letting it open so I kept on getting E.4.8. I have also replaced my o2 sensor last summer, due to it being lazy. The new one is Bosch Premium and when I monitor it through the on-board diagnostics it seems very active. It even warms up faster and lets the computer enter closed loop a little sooner than the old one. This one lets the computer enter closed loop around 65C, if I remember correctly, and the old one was letting it around 85-90C.

What I can't explain is why all of this started after the distributor removal. Prior to that I do not recall such hesitation.

I just thought of something. Is the distributor gear supposed to have some grease on it? I remember when I pulled it out the lube on the gear looked like grease to me, and not engine oil. I wiped it off, cleaned the gear and reinstalled the distributor. Maybe it wasn't grease, but thickened oil?!

[mc_marto]

Is the EGR valve original? It is possible an aftermarket replacement is allowing too much EGR flow, which is not a factor at higher throttle openings. There might also be a PCM calibration update to address EGR chuggle conditions. Does it idle smoothly? It might be helpful to measure the injector coil resistances (while hot) and perform an injector balance test; in Diagnostic Mode, individual injectors can be disabled and the resulting RPM drops can be observed.

The EGR valve is an aftermarket part, but I bought it last summer. I thought of that possibility, so I took off the vacuum supply from the valve and had it plugged up. Took it for a drive and there was no change. So, I think that rules out the EGR valve out of the equation.

I had the injectors measured a couple of weeks ago, and had 3 with resistance lower than 13 Ohms, but I don't think that automatically makes them bad. I think the coils might have lower resistance, but the injectors might be fully operational in the same time.

Good thing you reminded me about the power balance test. I will do that.

Otherwise they should be clean - I ran two tanks with Techron.

[adallak]

A lot of bad things happen after a tune-up.... That's why I call it screw-up. EGR-related hesitation has different pattern. Something in ignition system got messed up.

[mc_marto]

Here are some updates:

1. Checked the ICM at AutoZone - the guy ran the test 8 times and the module checked good

2. Checked the resistance of my new wires - very good; the insulation is good everywhere as well

3. Checked all of the spark plugs and as suspected the plug for cylinder 4 was fouled (my old wire for the same cylinder was burnt on the exhast)

The other plugs seemed fine but I cleaned and re-gapped all of them. It seemed like the pulsation at the said RPMs was less. Today, I re-adjusted the TPS, so it shows 4.1* at warm idle (specs from the FSM are 1.6 - 4.6).

Now the throttle seems a lot more responsive, but the pulsations are more noticeable. It is as noticeable as when I first felt them when my timing was way retarded. I double-checked the timing and it is where I set it - 10* BTDC.

So, I think the only possible thing that creates that low RPM hesitation is the spark plugs. Even though I cleaned them they are not perfectly clean, and the plug for cylinder 8 was worn. I did re-gap all of them, but that's the only thing I can think of causing my problem. The transmission is very smooth and shifts very well, so that could not be it.

I drive mainly in town, but my plugs, given they are double platinum shouldn't have gone bad so soon. They have been on the car for a little more than two years, and I have put only 15 k miles on them.

I think it's probably true that Cadillacs don't do well with other brands of plugs than AC Delco. The plugs I have right now are Autolite Double Platinum - APP605.

What do you guys think, am I right in my conclusion about the spark plugs?

P.S. I stumbled upon something on the internet. Someone was saying if the spark plug wires are routed too close to each other that can cause cross-fires due to inductance, but shouldn't that be applicable only if the insulation was bad as in old wires? Otherwise, I don't think it's possible to route the wires in a way that none of them are going to be close. What do you think about that?

[KHE]

Crossed plug wires can cause a misfire but as long as the wires are routed per the factory spec. you shouldn't have an issue.

I have no expierence using Autolite plugs in a GM vehicle - I have always used AC plugs in my GM cars.

[Texas Jim]

As KHE said...If the plug wires are routed exactly as per factory spec... they should be fine.

But if they are crossing over each other and touching each other, there can be cross fires.

if the wires do have to cross each other.. try to have them as close to a 90 degree angle as possible.

Do not let them run parallel to each other and be touching together.

[mc_marto]

Here is an update:

I just fixed my issue tonight. It was something really stupid that I never looked into until tonight.

I'm guessing someone had the distributor removed at some point. When they reinstalled it they did not rotate the base enough so the cap seats good. I'm sure I didn't screw it up because I had marked everything prior to the distributor removal, and aligned everything when I installed it. There is a small notch on the cap so it can go one way only on the distributor. I guess they did not notice that on time and later on just shaved the notch a bit, so it could slide over and look flush with the distributor base. When I replaced my cap I didn't notice the notch and just had it positioned the way the old one was. Well, that wasn't letting the cap be flush first of all, and second - the cap got cracked from me trying to tighten that bolt as much as the others.

I discovered the notch on the cap tonight, and looked for it on the distributor base. There it was but it was 180* from where it was supposed to be, so the cap can mesh with it good. Well, I loosened the bolt, rotated the base, glued the crack in the new cap, installed it, and re-adjusted the timing. Took the car for an hour drive and there wasn't a single hesitation. Just a smooth acceleration all the way through. I'm so happy that I finally found the problem. I just wish I would've found it much earlier, and not have spent weeks of trying to fix it and researching similar issues online.

I appreciate all of your help. Thank you all!

[winterset]

Do you trust gluing your distributor? - maybe just temporary - but you might want to invest in a new undamaged cap.

[mc_marto]

True. I would most likely put another cap on, but this one didn't really break - it just had a small crack which I simply filled with the super glue. It should be good, but like you say I might want to replace it. That's not a problem.