Part throttle hesitation or miss, optispark problem?

[aroethli]

I have a 1995 Roadmaster sedan, with about 160k miles on the clock.

About a year ago it started to occasionally miss when the engine was under a light acceleration load. This has now become much more common once the engine is fully warmed up, such as idling for a while or about 10 minutes into highway driving.

The symptoms are that under part throttle, the engine misses heavily.

If running at a constant speed, say with cruise control at 70mph, there is not apparent problem. But if I start to climb a slight grade and the cruise control, or my foot, starts to give slight part throttle... the engine will miss and buck. If I give significantly more throttle, I can "push" through these misses and the engine will smooth out. "Flooring" the throttle will also force it through.

Similarly, if I am idling for a while and get the engine good an warm, when pulling away under part throttle... say pulling away from a drive-through or into traffic from a parking lot, the engine will miss and buck. Again, giving heavy throttle will usually smooth it right out, but not always.

I took it to a local shop a few months back and they did say they measured/observed problems from the distributor output when the engine warmed up under these situations.

So a few questions:

1) Is the optispark bad, or going bad?
2) Can the optispark be fixed (cleaned?) in place?
3) Can the optispark be cleaned or repaired without buying a new one?
4) Could this be another problem that is causing the optispark to have issues, or appear to?



If I need the optispark replaced, I know it is expensive. I also think I recall that Delco no longer makes them. Knowing that, I don't mind spending more for a better aftermarket unit that is durable. What make/model optispark is recommended?

Thanks,
Al

 

[paart]

There are two general areas to "look at" that can cause the symptoms that you describe. The first is fuel, the second is ignition. From your description, I'd guess fuel---and that's a guess. The reason that I make that guess is from your statement that you can "push" through it with more throttle. Weak ignition issues are often dependent on engine speed and load, and "more throttle" typically will make things worse. But not always, as more throttle opening also reduces ignition advance, and richens the mixture (at WOT).

First, I'd run the onboard diagnostics from the console, to see if any codes are set. If so, that may pinpoint the issue. Check the hose and electrical connection to the MAP sensor, and the electrical connectors to the MAF and TEMP (at the front left of the engine). Check all the hoses and clamps from the air filter through to the throttle body, looking for possible leaks.

If you cannot find any fuel or sensor issues, I'd consider plugs and wires before suspecting a problem with the OPTI. I'd clean the ground connection at the front of the head, and consider replacing the coil (they are cheap), if it looks at all suspicious.

How was the "diagnostics" performed by the mechanic? Oscilloscope?
It's fairly difficult to pinpoint an ignition issue to the distributor.

IMHO 90% of OPTI issues can be corrected by replacing the cap and rotor, and possibly the vent hose assembly, although changing the cap and rotor does require removal from the engine. There is just very little else that fails in this distributor.

 

[wcamaddox]

Sounds like a throttle position sensor to me. Like it has a dead spot in the rheostat.

 

[aroethli]

First, I'd run the onboard diagnostics from the console, to see if any codes are set.

Thanks, are you talking about just an OBDII reader, or something else? I've heard that the HVAC controls can be made to read out various stats, so I didn't know if you were referring to that perhaps.

Thanks,
Al

 

[paart]

Yes, this procedure is listed under "stickies" at the top of the forum.
The "00" group would probably be the most pertinent.
'95 Buick is ODB1, although I think an ODB2 code reader would also work.

 

[grandpabuick]

Of course I don't have the optispark, but when my PA had the same sympoms it was the coil pack ignition module. Low revs in overdrive equals low ignition demand, more throttle demand and weak ignition can't cope.

 

[m00ntan]

4) Could this be another problem that is causing the optispark to have issues, or appear to?

The optispark is a sealed unit behind the water pump with two moving parts and tolerance designed to last 300,000 miles.

With 160K, you need to replace your plug wires, if that's not in your records. It's not easy, takes two people and a lift, costs $500. And you have to check all NINE wires are replaced and snug.

I know this because I had various suspisions about ignition gremlins. Finally got stuck on the side of a mountain with no tools, no time and no parts.

I mean to say, do this FIRST just because you must, then worry about opti spark.

I hope the wires resolve your problem.

 

[aroethli]

Sounds like a throttle position sensor to me. Like it has a dead spot in the rheostat.

Hi, can you tell me where the TPS is on the Roadmaster LT1?

Can it be cleaned, or just an easy replacement? If replaced, will the timing need to be recalibrated, or is it just a bolt-off/bolt-on?

I'm more convinced as this persists that it is something like this as it is just one dead spot and no misfires at any other throttle position.

Thanks
Al

 

[200OZ]

The TPS sensor is on the throttle body on the exact opposite side as the throttle cable bracket. The sensor is held on by 2 small torx screws (T-20 I think).... It's easy, just disconnect the battery, and swap it out.

Did you check the ignition wires, spark plugs, and what not? After reading your first post I'm going to say it's an ignition problem. The opti was designed to be maintenance free for 100,000 miles, with 160,00 on yours it may be time for a cap, rotor, plugs, and wires. The distributor itself is probably fine as long as the vent harness is in good shape.
A good vent harness is essential for a long lasting distributor.

Mike

 

[aroethli]

Anyone know the part# and approximate cost of a new TPS? I'm with you on the ignition parts as well, but if the TPS is inexpensive and is just a bolt-off/bolt-on affair, that might be a quick and inexpensive test to eliminate that as a possibility.

 

[DirtyDR]

I have the exact same issue with my new 96 Roadmaster sedan with only 74,000 miles on it and I could not have explained the issue any better. It was my fathers car and he bought it new so I have every receipt and I know he had the fuel system cleaned at the dealer at 55,000 miles so I was suspecting something ignition related also. I have just been waiting for some of the snow to melt so I can get out and do some troubleshooting.

When I picked it up from him a month ago he told me he just had the oil changed and the chassis lubed about 1,400 miles ago and when I looked at the receipts he was correct except he failed to mention that was in 2010 so the car has not been driven a lot.

 

[aroethli]

So here is an interesting update and observation: When I filled up with a tank of 89 octane fuel, the knocking (which now I assume is detonation, not missing) is either gone or greatly diminished. I still felt just the barely slightest rough accelleration I think on a couple hill climbs, but no audible knock and stumble as before under even lightly loaded part throttle.

Now, ths is only with 1/2 of the 89 octane tank full burned over a few days, and the outside temps here have been very cool at 30-50F. So I can't say if the problem will reassert itself with higher temps, but I should find out soon enough as we're due for some 60-70+F days soon.

So again, perhaps premature and I'll update after a couple more tanks of 89 octane, but with this observation does this provide any more information to help diagnose what the problem is?

Clearly detonation has a few classic causes: Ignition timing off, too lean fuel mix, hot spots/heat and compression.

Well, I know I haven't increased compression 🙂


Here's another tidbit, my temperature gauge quit working some time ago. It is apparently sent from the sender on the passenger head. I'm still looking to see if it is the sender or my gauge cluster, but suffice to say I suspect the sender or wiring connector and not the cluster since the temp needle moves through its range when I start the car.

So, I thought this sensor was just for the gauge cluster, but touching on one of the causes for detonation (heat), does this sensor input anyting into the computer for ignition/fuel based on head temp? If the sensor or its wiring is bad, could this be the problem or is this sensor unrelated to regulating ignition timing and air/fuel mix?

I'll update. I'm also about to walk out and try to get any codes from the HVAC.

Thanks
Al

 

[aroethli]

OK, checked the HVAC codes, and none so far. I'll try and watch it while I drive later as well.

 

[aroethli]

OK, an update after some additional observational testing.

While the weather was still cool (30-50F), I ran through about three tanks of 93 octane gasoline versus the normal 87-89 grade I normally run. While running the 93 octane, the "miss" just about went away, and was nearly undetectable. I could still feel a little hesitation on part throttle accelleration under load, such as up a slight grade, but the more significant "bucks" and "misses" were gone.

Now that the outside temp has risen to the low to mid 80s, the more noticeable "miss" has returned on part throttle even with the 93 octane fuel.

With this observation, I'm inclined to think that he "miss" is really a serious ping/knock situation given its sensitivty to octane and temp. What do you think?

If so, does this provide any better clue on where to start looking for the problem?

I've priced a new TPS(which was recommended earlier in the thread), and they are very inexpensive, so I may just get one for the heck of it. At 160k miles, it couldn't hurt.

But hopefully one of the experts here have a little insight with this bit of additional info instead of just throwing new parts at the problem and hoping for the best.


Thanks
Al

 

[DirtyDR]

Someone told me the EGR can cause the problem also but I haven't looked at mine either. Last week the check engine light came on on mine and then went out, it did this a couple of times over three days and then the fuel pump died. I do not know if a fuel pump issue would solve the problem and I haven't had a chance to drive it since I got the fuel pump in but I will take it out for a drive tomorrow and see if maybe the failing fuel pump was the culprit. I never did get any codes on the display when I checked even with the light on and I was going to but my PC based OBDII scanner on it but the fuel pump died so I did not get a chance to check it.

 

[aroethli]

Thanks, yeah I was just reading that on the Impala SS forum where they have a "sticky" for missing, pinging, surging. EGR is one of the... unfortunately many..... culprits that can apparently cause these symtoms. Although, the Opti seems to be very common too.

I posted this issue there too given the larger audience. Hopefully someone here or there may even have a few standard tests/diagnostics to recommend that would really narrow it down further.

At 160k miles, it probably wouldn't hurt to just replace things like the TPS (cheap) and the EGR (sorta cheap) and see. At 160k miles, they are probably both "wear items" 🙂

So I'll probably do so and see if they help.

Al

 

[DirtyDR]

Al,
I changed out the TPS on mine yesterday and took it for a good run on the Interstate and it seems to have fixed the problem. I am hauling the dirtbikes out to Moab tomorrow for a week so I guess I will see if that was the problem for sure.

Dana

 

[Large Car]

The temp sending unit will have a big affect on the way it runs,it sends a signal to computer to tell it how much fuel it needs.I just had to replace mine it was telling computer it was 160 below 0 so it was dumping fuel to keep it running.I imagine if even connector was corroded you would get bad reading worth a look it made big impact on mine.

 

[aroethli]

Thanks guys. I'm also thinking about changing the TPS adn EGR as they both are relatively inexpensive and wear items. I'm certain that both of mine are original, which means 18 years and 160+k miles. So even if they aren't the culprits/solution, it's probably just a good idea. I'll let you know.

I know my main temp sending unit is good, but the aux unit that runs the gauge isn't working. I don't know/think that the aux temp sending unit (in the passenger side head) has any input to the computer?? Some have said it does not.

 

[MrBuick (Magnum)]

The temp sending unit in the pass side head doesn't give any imput to the PCM. It's broken or not hooked up on about half of my cars. I've noticed no difference in how the engine runs.