GL 1200 LTD Fouling Plugs

[RockinRon]

A couple years ago I picked up an 85 LTD with 45k miles in relatively good cosmetic condition. With a new battery I was able to get the bike started. I replaced the rectified/regulator (no charging condition) and after synchronizing the throttles, was able to get it to run smoothly at idle cold. However, once the engine starts to reach operating temperature, it apparently doesn't lean out the mixture and shortly thereafter it dies and won't restart unless I replace all of the spark plugs. I managed to drive it once for about a quarter mile, barely making it back home, keeping the rpms up, while it backfired/afterfired (not sure what the difference is).

All of the spark plugs foul evenly. I replaced the Tw sensor and adjusted the throttle sensor (it did not require much change) and it made no difference in symptoms. Like I said, it seems to run okay until the coolant temperature starts to increase, then it starts to miss and eventually dies. No error codes register on the ECU. I have been using the shop manual troubleshooting guide. I doubt that the problem is associated with the dual systems (PBs, GL/GRs) since both banks are having the same problem. I checked the resistance of the Ns sensor (360 ohms-normal range measured at the ECU).

I suspect I might have excessive fuel pressure but I can't figure out how to use my fuel pressure gauge which has a screw-on fitting like a tire pump. The manual isn't very clear but I suspect the gauge is supposed to be attached where the 6 mm bolt (10 mm head) screws into the right side fuel rail. Is an adapter required?

The only other common component that I am suspicious about is the air valve, only because it appears to be temperature activated. Can anyone explain what the purposes of the air valve and the reed valves are and when they kick in?

Any suggestions are welcomed.

Thanks for reading!

[Rednaxs60]

Welcome to the world of an older FI bike. I have one as well, had a second one that I put back on the road after it sat outside for four years. Bought it and rode it across Canada back to Victoria and then sold it - long story. Had the same issue with the one I bought back east. Did some major work, and replaced the TPS with a new aftermarket one. In the mid range, lots of backfire and hard misfires, almost felt like the engine would stop dead. It was mentioned to me on another forum that the TPS was faulty, but since I had just replaced it, was not listening. Started cross Canada with the faulty TPS, and had issues especially when it was hot. Checked a lot of fuel items en route, but nothing seemed to work. Bike worked well at low and high RPMs, but not midrange. Did have a spare new TPS with me and when I reached Brandon, Manitoba, decided to change it. The newly installed TPS worked a treat, no more issue(s). These items are inexpensive (cheap) to make and the QA for this item is not very good.

Back to your question(s). Your symptoms are indicative of a faulty throttle position sensor (TPS). These are temperamental and when failing give the symptoms you describe. These items are affected by heat, and having the bike rode at a certain speed all the time. These can be in spec, but still be faulty.

The fact that the plugs are fouled - excess fuel - is directly related to the TPS. You will probably find that the TPS is close to out of range on the high side giving the ECU an indication excess fuel is required.

The Idle Air Control Valve (IAC) is not an issue here. It is electrically operated in parallel with the fuel pump. Fuel pump on, power to the IAC valve. It is a passive system and the reed valves are activated by cylinder vacuum. Here's a picture of the air system:

This system provides additional air on start, and during engine operation. When you close the throttle to coast, this system allows air to enter the cylinders to keep the engine operating. If it did not, the engine would starve for air when decelerating. This system is operating at all times when the engine is started.

Make sure the engine idles at 1000 +/- 100 RPM at idle warmed up. Cold should be upwards of 1500/1600 RPM then settle back to 1000 RPM. The fuel pressure will be approximately 35 PSI when operating and a little higher when static. You have identified the fuel pressure test bolt, and need a fitting to screw into this spot. Don't know of an adapter, I bought a fuel test kit for mine.

Good luck.

[RockinRon]

Thank you Rednaxs60 for your reply.

Do you know where I can buy a TPS?

It looks like folks have replaced the stock TPS with an after market one which may require modification to the throttle body involving some machining, which I don't have the resources to do. Some folks had limited success replacing it with a TPS from a Honda Prelude but it appears these have been faulty too.

Suggestion?

Ron

[Rednaxs60]

These are no longer available. The ones from an early model civic/prelude can work, but like you mention, quality is an issue. I am still looking for a good substitute. No need to modify the throttle body, just the TPS. Let me dust off the cobwebs and I'll get back to you on this. In the last throws of finishing painting my 1500 - taking all my time right now. Once I get the 1500 done, will be rolling the 1200 out and playing with the TPS as well. Have the external alt mod (poorboy) on mine that limits the space available for installation of a new TPS.

[ct1500]

With the plugs fouling so quick I would imagine a massive amount of fuel is getting to the engine. It would have to be blowing some black smoke out the exhaust and making your eyes water, correct? I understand the 1200 is CFI and if like TBI the injector(s) might be accessible to view in operation. If possible look at the spray pattern which should be very fine droplets in a cone pattern, no dripping. If not possible to view in operation then try to look at pattern cranking the engine over. Before getting too deep into things I always suggest a compression test on an unknown non-running engine.

[CountryXJer]

These are no longer available. The ones from an early model civic/prelude can work, but like you mention, quality is an issue. I am still looking for a good substitute. No need to modify the throttle body, just the TPS. Let me dust off the cobwebs and I'll get back to you on this. In the last throws of finishing painting my 1500 - taking all my time right now. Once I get the 1500 done, will be rolling the 1200 out and playing with the TPS as well. Have the external alt mod (poorboy) on mine that limits the space available for installation of a new TPS.

What year civic prelude would these be? TIA

[Rednaxs60]

anywhere from 1985 and later. All the same, even if you by from a USA supplier.

[CountryXJer]

anywhere from 1985 and later. All the same, even if you by from a USA supplier.

Thanks new owner of an 85 Goldwing 1200L since friday the 5/8/20! Just surfing and trying to get ahead of the power curve. Only issue at this point is no charge. Told by PO that the stator was bad, but as soon as it warms back up, I'll start doing the complete tests myself.

[RockinRon]

Thanks to all who are participating. I saw one post where someone suggested that the TPS from an 85 CRX was the preferred alternative; however, I was unable to find any sources for this particular part. I ended up buying a TPS for a Prelude/Civic but haven't had the inspiration to try retrofitting it yet. (Garage temperatures lately here in AZ have been >100 degrees). The new TPS uses 5 mm bolts to fasten where the LTD TPS uses 6 mm bolts so I would have to find a workaround for this problem.

Again, I have to stress that my problem starts as soon as the engine gets to or close to operating temperature, as if the ECU is not getting the appropriate engine temperature readings, or not compensating for the increase in engine temperature. I fail to see how throttle position affects this, but I'm no expert.

Could be excessive fuel pressure? Still can't figure out how to attach a standard fuel pressure gauge to the 6 mm port though. Is a Honda special tool required? Has anybody done this? If so, please share how. Thanks.

[Nolimit83]

Rocking Ron. Did you get the bike sorted out? Was it the tps or excessive fuel pressure? I am curious as to your findings. I too am having this issue and seem to find nothing but dead ends. Did you perhaps say enough was enough and got rid of the bike altogether?

[Rednaxs60]

Rocking Ron. Did you get the bike sorted out? Was it the tps or excessive fuel pressure? I am curious as to your findings. I too am having this issue and seem to find nothing but dead ends. Did you perhaps say enough was enough and got rid of the bike altogether?

Start anther thread about your issue(s). Are error codes generated by the ECU? If you think the issue is the TPS, there should be an error code, but not necessarily. Is there a stutter or "bog" in the lower speed range? Have you balanced the engine cylinder banks lately? Which plugs are fouling. A fuel injector clean, flow and leak test would be good maintenance. When you start the engine first thing in the morning, does it go to a fast idle with no throttle assist - starts immediately, RPM around 1500 then reduces as the engine warms up to operating temp, or does the engine RPM drop to around 800/900 RPM and with or without throttle assist goes to a fast idle? BTDT for all, some issues have eluded me even after 8 years.

Have modified several of the aftermarket early model Honda TPS units to fit. Also have to work around the external alt mod - not a lot of space left. The aftermarket TPS units need to have the actuating arm modified to engage the left side throttle plate shaft. The TPS case can be modified to work with the original TPS securing plate. Wiring is not hard to retrofit. If you do order aftermarket TPS units, get more than 1. New does not mean it is good, BTDT.

The TPS for a 1985 Honda Prelude is 16046-PK1-671. Check with your local Honda dealership, may have a NOS. I go to the local Honda dealership for parts when the bike shop cannot supply, lots still available.

Good luck

[Nolimit83]

Rednaxs60, it is not my intention to hijack this thread. Merely to find closure on it. My gl1200 LTD is showing the same exact symptoms that rockinron was experiencing on his bike. If there was a solution found, or if it was not, it would be beneficial to others knowing the outcome.. Even not having a solution can be helpful. There may be others that have an issue similar, and I find it daunting trying to find "the fix" when another thread was started without any direction for others to follow.

For any one reading this. Rednaxs60 is going hard-core in depth with the electronics, computer, and working of the fuel injection system on this bike. Even compiling what I would consider a book about the inner workings. Very knowledgeable.

So, I may start a new thread. For those reading and looking for answers. Click my name and search my posts. Until then, let us hope that rockinron found a solution and brings forth a well of knowledge that we may drink upon and grow together from.

[Rednaxs60]

Rednaxs60, it is not my intention to hijack this thread. Merely to find closure on it. My gl1200 LTD is showing the same exact symptoms that rockinron was experiencing on his bike. If there was a solution found, or if it was not, it would be beneficial to others knowing the outcome.. Even not having a solution can be helpful. There may be others that have an issue similar, and I find it daunting trying to find "the fix" when another thread was started without any direction for others to follow.

For any one reading this. Rednaxs60 is going hard-core in depth with the electronics, computer, and working of the fuel injection system on this bike. Even compiling what I would consider a book about the inner workings. Very knowledgeable.

So, I may start a new thread. For those reading and looking for answers. Click my name and search my posts. Until then, let us hope that rockinron found a solution and brings forth a well of knowledge that we may drink upon and grow together from.

Not going hard core with electronics-computer or the CFI system. I'm a frustrated FI model owner, and have been working on my '85 Limited Edition FI model for 8 years and have gone over it with a fine tooth comb with no real solution.

Only CFI component that is adjustable is the TPS and there is not a lot of wiggle room regarding TPS calibration - you can calibrate the TPS until the ECU creates an error code, then back off the calibration a bit. Do this for the lower and upper range and determine which is best - shouldn't have to but it's all we've got. The TPS can affect spark plug fouling, but not too much at the lower engine power range and idle.

The ECU uses Speed-Density fuelling at the lower power range and at idle. The PB (MAP) sensor(s) and inlet air temp (IAT) signals are used to determine the air density and with the engine RPM, a fuelling requirement is determined from the fuel maps for the speed travelled at.

The ECU also uses Alpha-n (Alpha for the throttle position and "n" for the engine RPM) to determine the fuelling requirements at the higher power ranges.

To foul the spark plugs, mostly at the lower RPM range and at idle, fuel-air mixture. Air supply is determined by the throttle opening and engine vacuum to draw in air. At idle and for deceleration, combustion air is supplied by the idle air control (IAC) circuit because the throttle plates are closed. Fuel is determined by the ECU based on various signal inputs to the ECU to match the air mass flow into the engine.

Since the ECU programming can not be adjusted/tuned, have embarked on a CFI system ECU replacement/upgrade project. The new ECU will allow me to tune the engine six ways to Sunday so to speak. I have had the new ECU installed/connected to the CFI system, engine running. It is proving to be a good learning experience. Started the base engine tuning this week, had started before but due to technical difficulties, had to regroup and start afresh.

It has been mentioned that the ECU components can degrade over time considering these ECUs are some 38 years old. Reasonable expectation. This is a picture of the ECU "guts". This ECU had a ground fault and is now unserviceable. I'm using this ECU enclosure to house the new ECU:'

This is the new ECU, wired and in the enclosure:

Slight difference from the OEM CFI system ECU.

Don't believe I have read about a definite solution to the issue of fouling spark plugs on these FI models, but do believe that most see an improvement and this is good enough.

Good luck.