This concerns a 1990 Formula with 305 TPI, 5 spd, N10 option. Car is in overall very good condition, but might have sat for a while. It jerks (bucks) at around 2,000 RPM in 3rd, 4th and 5th gears, but only if trying to maintain a steady cruise speed with light throttle. It idles well and seems to accelerate strong if you stay on it. Also, as soon as you lift off the throttle the issue goes away instantly.

I've read so many posts with similar sounding issues and have been consulting a Pontiac service manual. No codes; so, an intermittent issue for which the service manual tells me a number of things it might be and says "good luck." Most of the posts I've read seem to end without a conclusion. So far, I've changed the oil and transmission fluid (ATF), the rotor and distributor cap, spark plugs (correctly gapped), spark plug wires, fuel filter, and TPS. Each has seemingly reduced the incidences of the bucking/jerking issue and have increased the driveability; however, the issue remains.

Hooked up a new Hobo Freight fuel pressure gauge to the Schrader valve. Without the engine running, but after turning the key (to make the fuel pump run), the fuel pressure is right at 40.

I have hooked up a computer to the ALDL and have been logging data via TunerPro. The BLM sometimes goes all the way up to 160; however, the Integrator (INT) is often much lower. For example, at start-up the INT is 128, but the BLM is 150. And at the end of a recent logging session the INT was hovering around 128, but the BLM was 160. The BLM seems to almost always be 20 to 25 points higher than the INT.

I've tried to research what would cause the BLM to be so much higher than the INT, but everything I've read says the INT drives the BLM and they should be roughly equal, which does not seem to be true in this case. Any ideas why these numbers would be so far apart? And does the cause of this difference potentially explain the bucking/jerking issue? Help much appreciated. Thank you.

 

The O2 sensor drives the Integrator and the Integrator drives the BLM. Integrator is thus Short Term Fuel trim and the BLM is Long Term Fuel trim. In a perfect world, 128 is Zero fuel adaption. The ECM tries to keep the 02 sensor cross counting over the target voltage by shifting the Integrator. The ECM tries to keep the Integrator at 128 by shifting the BLM. 150-160 BLM means the ECM is correcting for a fairly substantial lean condition. The lean condition could be real or false. Low fuel pressure, vacuum leak, bad/dirty MAF, wrong or clogged injectors or low fuel pressure from a plugged filter or weak pump would cause a real lean condition. A misfire pumping air into the exhaust, an exhaust leak or an air pump diverter valve that keeps pumping air into the manifolds in closed loop would cause a false lean condition.

 

Thank you for this information. For purposes of attempting to pick a direction of fixing a "real" vs. "false" lean condition, would it help to know that when I changed spark plugs they looked very clean (almost pinkish in tone). And, a prior owner installed an MSD 6A. Two other things I've noticed that may or may not have any connection: (1) the gas gauge won't go above 3/4 (even when it appears the tank is filled); and (2) after the car warms up there is a slow and intermittent ticking sound that seems to come from the exhaust somewhere before or near the dual cats (N10 option). I believe, as a 1990, this car has a MAP instead of a MAF. I did change the fuel filter, which might have helped a little, but have not gotten into the more involved projects of pulling the injectors or dropping the tank. Wondering if any of that suggests I go one way or the other? Thanks again.

 

At startup, the INT will stay at 128 until the engine gets into closed loop. After it gets into closed loop and the ECM starts reacting to the O2 sensor, the INT will then start to oscillate around.

Since the O2 is on the driver side, the ECM only reacts to what the O2 sees on the driver side. Which means whatever is driving the 160 BLM could potentially just be on the driver side, and not something that's affecting the entire engine. 160 is essentially pegged lean since that's the limit on what the ECM is allowed to correct in the calibration.

One thing you could do is look at the spark plugs again if you've put some decent mileage on them. If you see that one cylinder on the driver side looks a lot leaner than the others (i.e., much whiter), it'd be an indicator that particular cylinder on the driver side is the culprit (which could point you to a malfunctioning injector, possibly leaky cylinder due bad piston rings, valve seal, etc).

However If all the plugs look very white, then it's something that would affect the entire engine... maybe a sensor that's reporting bad information, or one of the things that Fast355 mentioned about exhaust leaks and stuff, faulty ignition coil, etc.

One basic thing you could do is check your ignition timing.. supposed to be 6 deg BTDC with the EST connector disconnected.

 

Thanks. I'll probably start with the remaining basic item. If and when I'm able to get this solved (fingers crossed and much time later at my rate of progress) I'll post it in case it helps.

 

Update: I disconnected the MSD 6A and reconnected the stock harness to the coil, have replaced the coil (frustration buy) and have checked the timing--it is exactly 6 degrees BTDC with EST disconnected. Another symptom (potential clue) is that after the engine warms up and the idle drops down to 700 rpm (T-5 manual), there is a distinct, but slow (like every 1.5 seconds) tap - tap - tap sound that starts / stops / starts only at idle and goes away as soon as throttle is opened. It is slower than a valve and rod knocking, does not change in tempo, does not do it when cold, comes and goes, and goes away (or not audible) when driving, except when stopped. I have not been able to pinpoint the source. It sounds like the top of engine (305 TPI), but I'm not sure yet. I've read that a bad/dirty EGR valve could cause misfires at the specific RPM level (1,850-1,900), which is cruising at light throttle. I've also read about a bad purge solenoid and/or canister?

Anybody encounter a situation like this or have further suggestions? Thank you.

 

And you know this ... how???

Have you tried adjusting it until the motor runs THE BEST it POSSIBLY can?

EFFFF whatever some "mark" says. In the words of one of the all-time GOATs in a totally unrelated field, and had he been working on YOUR car, he would have said:

"If it RUNS good, it IS good".

Conversely if it DOESN'T run good, it ISN'T good.

Try advancing it. If it RUNS better, ... it IS better. No matter what YOU think some "number" is telling you. I can assure you, after enjoying some 50-odd years in this hobby, YOU don't know better than THE MOTOR does, what "number" makes it run better. Put aside your ego and GIVE YOUR MOTOR WHAT IT WANTS instead of what YOU think it should have.

I suggest ignoring the "tap" until the motor RUNS as GOOD as it possibly can.

 

[QUOTE=sofakingdom;6516315]And you know this ... how???

Using a timing light with EST disconnected. And when EST is connected, it advances the timing to 12-14 BTDC. Motor runs well, except for the bucking/jerking right at 1,850-1,900, and the tap tap tap....

 

I had a bucking issue at the same RPM years back. It would cruise pretty much ok but when I tried to accelerate it bucked like a bronco. If I really tried to accelerate it backfired, and backfired so hard that it blew out the header gasket. Turned out my ICM under the distributor cap was done. I changed that and all was well again.

 

And you verified your timing "mark" accuracy, right? You have the only one that's accurate?

 

Sorry, I'm not certain what you mean. I can see all the marks on the v-toothed gauge above the harmonic balancer and a prior owner put white paint / white out next to 6 degrees, which is right where the timing light flashes for base timing. When the computer (EST) is hooked back up the timing light flashes above the gauge, indicating it is doing a good bit of advancing even at idle. Admittedly I am just learning about this, but it seems like base timing is not the issue. I suppose the ECM could be going haywire and causing an issue at the RPM range where it bucks and jerks, but the loud tap tap tap sound at idle seems like a clue worthy of investigating.

Regarding Edge's comment, I did change the ICM recently, hopeful that would solve the issue, but it didn't.

Thanks.

 

Could the loud Tap be picked up by the knock sensor? That would pull timing back.

 

The timing mark on your crank damper isn't on the crank damper. Rather, it's on the inertia ring, which is connected to the engine ONLY by a thin blob of SPOOGE. Inertia rings well and widely known to break loose and move, taking the "mark" along. Once that happens, the mark LIES.

If you haven't verified independently that your mark still lines up with ACTUAL TDC, then you have no way to know whether it's accurate (tells the truth) or not. Any "numbers" you get from it are suspect.

Yes of course the ECM advances the timing considerably when re-activated. Typically the spark advances around 20 - 25°. The fact that this is happening according to the plan is in no way an indication of ANYTHING about your base timing.

 

To your point sofa. There's the factory one compared to the new one.

 

This car is in very good shape, bone stock, and there is no indication the timing gauge has moved. It's on the front of the engine and it remains firmly attached. See photo. Seems like the probable answer to "is the base timing right" is "yes" although that can't be 100% definitely or independently verified. Nonetheless, in the interest of time, if the engine runs well at idle and all RPMs, except 1,850-1,900, what is now the most likely culprit--EGR? Will finding out that the base timing is actually 4 or 8 degrees BTDC, which is changed when the EST is hooked up, change the fix? Or the tap tap tap? If so, can you explain how? Just trying to get some help on fixing this from anyone who has dealt with this before. Thanks.

Timing mark on front of original LB9, circa 1990 (made in 1989).

 

Beautiful.

Is it accurate?

The thing that moves is not the "tab", which is reasonably securely welded to the timing cover. It's the MARK, on the crank damper.

If it has moved by the outer inertia ring separating from, or even tearing, the rubber that glues it to the hub (you can see the rubber spooge in your last pic) then the mark no longer tells the truth. No matter how pretty the aftermarket timing tape is.

Can't comment about the tapping; butt if the timing isn't what you think it is, or worse yet if it's sufficiently wrong regardless of any "think", the motor isn't going to run right.

 

@Rojo17 look at my pic. That's what sofa is talking about. Look how far my timing mark moved. The rubber broke down between the damper and outer ring. If I lined that old mark up I would be off 17 degrees base timing ATDC. If you are concerned about timing, it's best to verify it. Without removing the cover, the best way would be to grab an old spark plug, smash the porcelain out, and find a bolt that you could jam in there. Insert that into the cylinder for use as a piston stop (you can buy these on summit) COMP Cams 4795CPG COMP Cams Piston Stop Tools | Summit Racing. or amazon. Then hand crank the motor incredibly slow until the piston stops at the bolt. Do not dare crank past that point. That's going to be TDC. Make sure it's TDC on the compression stroke first, using your finger. Your timing mark should be at the 0 mark on the tab. If it's not, then you'll know the ring moved.

 

Sorry EDGE, got your pics mixed up with the OP's.

 

Thank you for the information @EDGE and @sofakingdom. I'll keep working at this and hope to soon find a solution. If I do, I'll post the resolution here for the good of the order (I've seen so many threads that seemed like they might have an answer but then end with no solution or conclusion). Cheers.

 

My friend had a similar (bucking bronco) issue that was caused by a lean condition. He replaced his injectors and the ECM needed a tune for them. If your car has sat for awhile as you said maybe your original injectors might have gotten clogged and not spraying enough fuel. Also is there a reason you did not start the car with the fuel pressure gauge attached? The FPR works on vacuum so the pressure will drop to 33-35 once the engine is started and idling. If you remove the vacuum line while idling the pressure should go up to 43-45.

 

I had to remove the MAP sensor, located right above the Schrader valve, just to attach the fuel pressure gauge. I was afraid to start the engine without the MAP sensor connected. I've tried to find and bought an -4AN adapter to the Schrader valve, but so far have found nothing that will fit in this tight space. I need a 90 degree connection that will swivel, but can't find one. This engine is not easy to work on.

 

Updates: Well, still note sure whether this is fixed. I hooked up the fuel pressure gauge by moving the MAP sensor and the pressures before start and at idle were good. I then embarked on replacing the injectors and finally completed that…. Unfortunately, that didn’t solve the issue either. Then I replaced the MAP sensor and fixed the broken clip on the connector. No dice - still bucks at around 2,000 rpm at light throttle, which is pretty much cruising speed in 3rd, 4th, and 5th gears.

Lately, I dug into the ECM. Saw a prior owner had installed a modified memcal with an aftermarket chip, hand-labeled #2 on top of the blue cover. I found the right OEM memcal on eBay and have installed that but haven’t been able to drive it due to weather. Hope it works as I’m running out of things to change.

How does one test whether the air / smog system is putting too much air into the exhaust? And could that cause the bucking at light throttle (but go away as soon as you let off or mash the throttle)?

 

Some people are just IMMUNE to advice and COMMON SENSE.

Good luck with that.

 

I've done a lot of reading on this site and studying of the Pontiac service manual on how to resolve this issue, as well as consulting with a certified mechanic. So, I'm all ears if you think I'm missing the boat here (which is I what I'm positively taking from your post).

 

I believe couch potato was referring to his timing advice (which was good advice). Not seeing any indication that you addressed that yet. Pretty easy and free to verify and adjust, vs throwing money at the problem (which you still might have to do eventually anyway after doing the free stuff).

 

Thanks. I did check the timing and the engine runs very well, aside from this intermittent issue. Per data logging (Tuner Pro), the engine has a lot of advance. The issue is only intermittent around 2,000 rpm under light throttle. The data (BLM) indicates this is a lean condition issue. Knowing that, would timing still be a fix? Based on what I know that doesn't make sense to me. I would think if the timing were off it would effect how it runs during idle and accelaration. Since I've already checked the timing and the inertia ring / damper appears to be fully intact, is it still worth finding TDC and confirming the timing?

I'm here to learn, for sure. I just have to be strategic in what I tackle b/c I have very limited time due to work demands.

I appreciate all the advice and will track down TDC if those more wise than me think it's still a potential fix. Cheers.

 

Still immune to advice and common sense, I see.

I didn't say "check" the timing, and then I told you WHY "checking" it is USELESS. I advised you to ADJUST it. Specifically, to ADVANCE it. Real simple. If it doesn't help, you can put it right back to wherever it was. If it does help, then, ... it helped. Takes longer to test-drive than it does to make and/or unmake the change.

No amount of "checking" is of ANY VALUE WHATSOEVER if the timing mark system is out of whack, as fully detailed above. Since it almost always is, "checking" is ALMOST NEVER going to be useful.

Since nothing else has been, it's still a possibility. What was the immortal Sherlock Holmes line... something like, when you have eliminated everything you can, whatever remains, however improbable, must be the truth... of course, in this case, "must" is a pretty strong word, so I would amend it to "may".

Calling people who try to help you a bunch of names isn't going to make your car run any better. Follow the advice instead of acting a know-it-all and making no progress as a result.

 

I haven't called anyone any names, don't know where you got that from. Being respectful and just trying to learn. I've tried to get at the hold-down bolt on the distributor to adjust timing, even bought a distributor wrench, but so far it has been impossible for me. So, it is not as easy as other things and this is not an issue of ignoring advice. If you have any advice on getting to that bolt, please let me know. I've read posts on that too, but nothing has worked so far. Thanks.

 

I usually use a 9/16" box-end wrench with a large offset to break it loose, then a short (like, 4 - 5" long) wrench to loosen it further and then re-tighten it. It doesn't need to get the full gorilla treatment, so don't worry about tightening it until you hear it screaming for mercy. That short wrench will do just fine.

Never have had much luck with a distributor wrench myself.

Sorry, it wasn't you that called me names, it was somebody else. My bad.

 

Thank you for all the information @sofakingdom. I appreciate your help.

I tried using a racheting wrench with flexible end, as shown in the photo here (sorry it's a little fuzzy). This is the first tool I could actually get on the hold-down bolt, but because of the EGR solenoid and coil, on 1 side, and the fuel rail, on the other side, I can't move the wrench unless I take the coil and EGR solenoid off. Only other thing I could think to use is a crow's foot wrench with long extension. I guess I'll buy one of those. I wonder if, because this is a 1990 (which seems to be more jammed up with stuff than some prior years) it is harder to get at? Do I just have to disassemble to adjust it?

 

More likely the problem is simply that it was overtightened.

Yes as time went on and various devices were added or changed, it got harder to access things that are buried deep. However these things are often very easy to remove for long enough to do what you have to do. The EGR solenoid for example is help on by only one small bolt, and will work fine unbolted as long as its frame is grounded to the engine (alligator clip lead such as https://www.harborfreight.com/18-inc...ads-66717.html for example). But, once you've got it broken loose, you don't need to re-tighten it in gorilla mode; 12 ft-lbs or so is plenty to hold it still. In fact, if you experiment with the tightness a little, you can find where it's just barely tight enough to mostly hold still but you still just barely turn the dist by hand, such as to allow adjustment; once you find that point of tightness, just tighten it a little bit more, and voilą! you've found the "enough but not too much" level.

 

Well, I'm grateful to finally be able to report that I appear to have found a solution to my driveability issues. In the event this might help someone else, I'm posting this conclusion to my saga. In short, after doing all the regular maintenance items and then trying to run through almost all of the driveability diagnostics in the 1990 Pontiac Service Manual, as well as changing a number of parts, as well as finally figuring out what tool would most easily get to the distributor hold-down bolt (a 9/16 flare nut crow's foot wrench -- see photo), it turned out to be the PROM in the ECM. As mentioned, a prior owner installed what appears to be a modified PROM with an aftermarket chip grafted in, on which they had written "#2". This implies there is a #1 floating around somewhere, but it did not come with the car when I purchased it. It took me a while to figure out what GM PROM it was supposed to have and luckily I was able to find 1 (and only 1) such PROM on eBay. (If anyone needs more information on how to do this, feel free to reach out and I'll dig up my notes. Tunercat.com was a big help, particularly since my car is a 1990.) In any event, as soon as I installed the new PROM, there was no more bucking, jerking, or back-firing, and now I can drive it around town and cruise, which is great. Since then, I have replaced the fuel tank vent valve with a reproduction from Hawks Motorsports, due to what seemed like excess pressure in the tank and a boiling sound coming from the tank, as well as difficulty restarting when warm. And I fixed a headlight issue I believe was caused by one of the pins in the connector not correctly engaging with the prongs on the headlight. Now I need to figure out the tapping sound at idle after warm-up....

I plan to take it for a longer trip this summer and flush out any more issues.

Crazy to think that even a car of this vintage, with a SBC and distributor, would be so dependent on a computer chip. Thankfully it was pretty easy to replace.

Thank you for all the comments and feedback!