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86 Omni GLH-S Boost boost tops at 9PSI and idle slightly high

1886 Views 42 Replies 7 Participants Last post by  luckky3000
Yes I think it might be a vacuum leak, but...
Bought the one owner GLH-S 53,000 miles at Mecum last month. Took it to a mechanic who rebuilt the CV axles (boots were torn), replaced the plugs, replaced the thermostat,confirmed timing is correct, checked the fuel pressure, replaced the pcv valve hose (was a patch job leaking). I have run two complete tanks of premium gas though the car in the past month both times using injector cleaning additive. Initially car did 8.33 0-60 with older bald tires (after tune up) and 16.33 1/4 mile at 86.19 (two different runs on same day).

Bought BRIDGESTONE 205/50R15 POTENZA RE-71RS tires and took it to a local alignment shop that does older cars and race cars. Had them look the car over as well and looked for vacuum leaks and did a smoke test but did not find any leaks. Did a compression test and all 4 cylinders were at 120. Air cleaner was not replaced but looks new. Two codes were present 42 and 47. Mechanics said the auto disconnect was working and the low voltage might have been from kissimmee where they disconnected the battery while it sat and reconnected when they moved it.

When cold, car idles at 1,200 and is sometimes a little rough. After it warms up it idles between 1,500 and 2,000. I installed a harbor freight boost gauge just before the map inside the car (for diagnostics). The boost gauge never goes above 10 (I believe it should go to 12) and that might explain the slower 0-60 and quarter mile times. After the tires the 0-60 is consistently 7.8 seconds. I understand it is hard to make a stock car match what car and driver or Motortrend list as their time of 0-60 in 6.5 seconds, but thought I should be closer to 7 seconds than to 8 seconds especially with modern performance tires. Launches were on asphalt not concrete.

I have repeatedly tested the vacuum system for leaks with soapy water, propane, brake cleaner etc with no leaks discovered. I removed, inspected and reset the Idle Control Valve and attempted to adjust the idle with the screw but could only get the idle to go up not down (the valve is the newer one not the one with the 3 gears and looks to be original). I have traced each vacuum line and have contemplated replacing it all with new vacuum block and hoses but the car is a nice survivor and trying to keep it as original as possible. I have checked for leaks around the intake and valve cover with no luck.

One peculiar note. If I remove the cap from the factory vacuum harness assigned to a boost controller (which was not installed in this car) creating a vacuum leak the idle barely rises 50 rpm and only for a quick second. This is why I continually suspect there is a vacuum leak, but not sure if the idle should jump higher if I do this. There is vacuum on the line when uncapped.

The car is fun to drive and pulls hard but it seems like it is below where it should be. It's a cars and coffee and car show car not a daily driver. A far as I can tell everything is stock and the car sat in a Dodge Dealership car museum for 20 years plus but was driven during that time. There is one spliced wire repair going to the AC compressor but nothing out of the ordinary. The valve cover does have a very small oil seepage above the number 4 plug but not able to detect an actual leak. The mechanic both thought it was not causing the issue.

Any guidance is greatly appreciated. I have searched the board and tried just about everything I have seen before posting for help. The only thing I have not tried is replacing the fuel injectors, but the car does run strong except for the high idle and possibly the boost being slightly low (which might be the cheap harbor freight gauge).

Here is a video of a cold start. Temp here is 61 Degrees on You Tube:
It actually starts rough and drops to about 500 rpm before returning to 1,200 to 1,400 rpms. It has started rough a couple times previously but usually does not so possibly a symptom.

Here is a shot of the 1/4 mile dragy result:
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Engine compartment:
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Idler valve (AIS I believe):
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Is the logic module a stock module or a MP module? I would unplug the red and blue connectors @ the LM and look to see if you see remnants (calcium deposits) of water in these electrical sockets. Don't be overly bummed if you have a bad factory LM. They were pretty crappy anyway as far as driveability. The MP ones off better performance, driveability, and mileage.
The stock correct LM should be marked CS1017 4419957
So question about the LM Stage 2 module. I see some people add the Grainger valve to get boost to 14-15 with the stock LM. What are the advantages and disadvantages of getting the LM Module vs just the Grainger valve? I won't be doing any mods until the base issues are resolved.
With increased boost comes increased torque. CS was concerned enough about the somewhat fragile a525 tranny, that they limited the full boost pressure until like 4000-4500 RPM and up. They needed these powertains to survive long enough to make em through the warranty period. The marketing people decided to try to sell that a good thing, and bragged about it on the window sticker, calling it a "sloped boost curve" (lol). So, adding more HP and torque is a two edged sword. Increased performance with increased load on an already suspected weak tranny. Put enough HP through a a525, and they will come apart, sometimes with spectacular results. I know first hand, as I've busted four of them in 20K miles on a GLHT.

With LM controlled boost, if the detonation sensor detects pinging, it will decrease timing and boost. Also with LM controlling things, the boost pressure will come on more gradual because the wastegate is already starting to open (slowing down building boost) as early as 4-5 PSI. You don't get that big torque spike you get with using a Grainger valve to control boost. With the GLHS module, they neuter it even more, but it prevents the tranny from damage. It also prevents the car from getting wrapped around a tree, making sudden lane changes, etc. when the boost hits hard.

With a Grainger valve installed, the boost will build much quicker (even at part throttle). It does this because the Grainger valve effectively prevents the wastegate from cracking open until near full boost is seen. Resulting in dramatically improved part throttle performance. This is also a two edged sword as the downside is much more load on the tranny because of the increased torque at a lower RPM. The LM no longer has control of boost, so it has lost it's ability to regulate boost if it senses detonation.

Many times when installing a Grainger valve, you will run into another roadblock if you keep increasing boost. The LM will have a preset overboost shut-off. This preset shut-off will vary from one application to another, and even one LM to another. Then to disable or prevent that, you have to select and install a MAP sensor zener diode that prevents the MAP sensor from seeing the actual pressure it is running. This is a very slippery slope as you can get bitten quickly with making this decision to run a zener diode. From there it is all downhill chasin your tail finding the next thing restricting performance without hurting the engine or tranny.
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Thank you. I am already contemplating the 555 upgrade people talk about on the board. Based on this and other posts here I would consider the stage 2 LM as far as an upgrade. With that being said might look for a GLH-T with a blown motor and little rust and build it from scratch.
When you do a little seachin, you're gonna realize that finding a rust free GLHT (even without one with engine blown up) is harder than finding a 86 GLHS. Very few people took care of or stored GLHT's, compared to GLHS'. The last really nice GLHT that sold on BaT brought about twice what you spent on your GLHS!

If you do decide to put a a555 in the GLHS, you'll need to decide whether to have it a cable operated shifter, or a rod operated shifter. Neither are direct bolt ins, but both are doable if you know your stuff & do your homework. Also do some research on the failure rate of intermediate shaft on the a520/a555's. They have their share of issues too.
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You can also install a 87 TII LM by adding in an external MAP. That is adding three wires and switching another two wires between the red and blue connectors. They're more plentiful, they're gonna be more inexpensive, and they'll be a big performance increase for your application.
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Obviously erratic idle speed is caused from having a 2 wire AIS LM connected to 4 wire AIS. You basically have a log manifold (pull through) LM trying to control a blow through idle circuit. I'm sort of surprised it runs as well as it does.
Glad to hear your GLHS is back running properly. Now if this is the only thing that previous owners and mechanics have screwed up, count your blessings!
Seemed like to me you have your choice of $125 clutch and pressure plate assemblies, or stepping up to $375-$450 clutch assemblies from one of our two main vendors. Doesn't seem to much in between.
On the cheap end the LUK 05-044 is like a stock replacement. Maybe a slight upgrade as I think the disk is 232mm instead of 228mm. That part # may have been superseded a couple times. I remember seeing one on Ebay a short while ago. If your planning future performance upgrades or plan on putting a far stickier tire on it, I'd suggest upgrading to one of the SPEC, or TU's clutches. I've also had good luck with Clutchnet clutches. General consensus is their disks are great but their pressure plates are nothing special. I have had zero luck with any Kevlar disks. The 3/4/6/ puck disks will be much more grabby than a stock organic disk, keep that in mind when making a clutch choice. It is very subjective as to what is tolerable and what is not for a street vehicle.
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