Turbo Dodge Forums banner

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

1887 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:
Azure Rectangle Font Material property Screenshot

Engine compartment:
Hood Car Vehicle Motor vehicle Automotive air manifold

Idler valve (AIS I believe):
Wood Auto part Engineering Gas Household hardware
See less See more
1 - 7 of 43 Posts
Do your heat and a/c bend doors work properly? I ask as they are vacuum actuated and the connector behind the control panel is VERY prone to crumble and cause vacuum leak.
Just watched video and saw 18" vac. That is about what my 87 GLHS had so that does not seem to far off. I say had, as it now has 20" vac after my head change. As for AIS, mine has the same as yours with the spring plunger style. Does car have stock exhaust or been modified? What about spark plugs, plug wires, distributor caps and rotor? Fuel filter changed out?

When I bought my 87 GLHS it would make boost using a manual boost controller, but only 5lbs when hooked up to the solenoid packs on the right fender. I found the green nipple going to the waste gate solenoid was barely fitting on so it leaked. Since I could not shrink the green elbow, I wrapped the solenoid nipple with electrical tape making the nipple a larger diameter and then cut the excess tape off that stuck above the solenoid nipple. I did this with all my solenoids elbows and now all fit tight. Oh, and now the car hits 14lbs boost (running MP Stage II LM).

Will be interested in what you find with the idle as mine has twice jumped to about 2k idle for a while and then returns back to normal. No codes. I assumed it is the AIS so cleaned it last time I had the engine out, but mine is so infrequent only time will tell.
See less See more
BTW a smoke machine is cheap if you're curious. I think I paid like $80 for mine, but you have to have a separate air compressor to supply to it.

ANd congrats on the GLHS! Did the car stay in FL?
What is the MP Logic Module?
Mopar Performance LM that I believe has revised spark and fuel curves as well as allows boost go to 14lbs as long as.it sees no problems. Hp I think is also upped to 200hp.
They pop up, but as you mentioned not everyday. Also not is the 86 MP LM is the same as 87 MP LM. Hope someone can answer that for you.
As mentioned MP State II LM has revised spar and fuel curves. Stock LM with a G valve will not have that.

For low boost ypu could have a boost leak in a hose, bad turbo, leak at wastegate solenoid etc.... G valve can take out the solenoids or LM from your boost problems.
Fwiw I am in the middle of the 555 swap in my 87 GLHS. I also added the chromoly bearing support plate to the trans while it was out. Hope to have it all back together this next weekend.

I did the swap to preserve the numbers matching 525 on my car.
1 - 7 of 43 Posts