Ram Jet idle drops when slowing/stopping

[DomSmith]

I have not been active on the forum much lately but am hoping to get out on the trails soon and am looking to solve a problem I have been putting off with my recent Ram Jet install.

I occasionally have issues with the idle dropping too low when slowing or stopping. It drops down to 200-300 and almost stalls. Every once in a while it does. Here are a few things I've determined:

1. There are no codes on the computer (MEFI 4b with o2 sensor).

2. It does not seem to be affected by brake vacuum.

3. It doesn't matter whether it is hot or cold.

Anyone else have this problem with a PFI motor?

Any suggestions would be appreciated.

Thanks.

 

[John Smith]

Did you rule out number 2 with a vacuum gauge in the cab while braking and not? Or does it always happen when you let off the gas, brakes pressed or not?

 

[DomSmith]

It happens both without using the brakes (just slowing when out of gear) and when using the brakes.

 

[lone gunman]

does the ram jet make use of a VSS (vehicle speed sensor) ? i know my TPI (tuned port injection) does and if you dont use one it can cause the situation you describe on my TPI.

 

[greenfox]

Do you have the a/c sensor blocked?

 

[DomSmith]

Gunman-
No VSS on the RamJet. The one sensor I may need to troubleshoot is the IAC (Idle Air Control Valve).

Green-

Not sure what you mean by the A/C sensor unless it's the IAC above.

 

[Sharp38]

The IAC is probably your problem. It's a little ratchet motor that bleeds in air to control the idle speed. I would double check and make sure that the seal is good around the IAC. It also may either be bad or have some crud in the air vent that is preventing it from doing it's job. Don Home Page Wolf Creek Rod Works

 

[DomSmith]

Thanks Don. I saw a post online about an adjustment for the IAC that cured the problem for a guy with a Vette but I don't see any screw adjustment. Maybe inside the Throttle Body?

I'll try to look there after taking off the filter.

 

[DomSmith]

OK, after taking off the IAC and making sure it was clear of debris the problem persists. Stopped by TLC in Van Nuys (very sweet setup there) and it seems Jay may have helped me find the solution?

With the rig running and parked, if you repeatedly pump on the brakes and then hold them the idle dips after 2-3 seconds and then comes back up to normal. I'm not home to look at the FSM for a booster test and my vac gauge is there as well but I'll read up tonight.

Sound like a booster or is this normal behavior.

By the way, all the guys at TLC were very generous with info and their time.

 

[UNO_RACING]

Did you check your timing. Also it could be the TPS as other's have mentioned. The suggestion to hooking up a vaccum gauge is a good point (15 to 20 vac is good). Also, how are your valves? Tight valves could cause problems with vaccum. Good luck.

 

[DomSmith]

OK back on board with trying to resolve this problem... Anyone heard of or used a "IAC signal tester"? I think this may be my culprit. Middle of the page...

GM RamJet crate engine diagnostic and tuning tools