Nucking Futs

Howell TBI install tach signal question.

10 posts in this topic

Hi all, found this little gem of a forum after trolling the internet for some answers. Seems like a lot of smart people (smarter than me at least) here, so thought I would pick ya'll brains.

A little background. I am installing a Painless (oxymoron) 10150 Direct Fit Harness to my California 1984 CJ7, plus installing a Howell TBI kit, with a MSD Street Fire CDI. There are some issues that need to be clarified to me, and may benefit others too. So I will expose my ineptness for the benefit for all. I will limit this to one pressing question for now.

Per Howell instructions-"After the grounds are secured, route the labeled purple and orange wires to the starter solenoid on the right hand fender well or to the starter. These wires each have a lug that attaches to the stud on either side of the solenoid or starter. (FIGURE 11) The orange wire is battery power and connects on the battery side of the solenoid or the positive post of the starter. The purple wire connects on the opposite side of the solenoid or to the starter solenoid post of the starter. Connect the labeled white wire to the ignition coil negative terminal to pick up a tach signal. NOTE: Multiple spark discharge ignitions require a special tach filter to function correctly. Contact Howell Engine Developments if you need one."

So the inept question time.  I have a Multi spark ignition. I am attaching the Howell Orange and Purple wires to solenoid per instructions if that matters.

However, should I purchase the special tach filter (as recommended by Howell) to connect the tach signal for the ECM from the CDI, or can I just connect the WHITE WIRE to the negative (-) terminal of the coil and be golden (or is that a bad idea)? If coil connection is ok, is there a benefit to connecting to the CDI over the coil? The only thing I can think of that may be an issue attaching to the coil is the 430 volts the MSD is sending to the coil and that being an issue! Kinda thinking the coil connection is a bad idea with this CDI (or any CDI).

Thank you.......

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Please share, Nucking Futs...I was going to suggest that you take an actual voltage meter reading at the negative 12V coil lead (primary posts).  

With an MSD AL6 box, I have used an MSD tach filter on a breaker point ignition for a vintage Jeep 4-cylinder application.

Let us know your "fix", and thanks for joining us at the forums!

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Sorry for the delay, several home projects that needed my dire attention. 

My fix was to directly connect the ECM to the tach signal from the MSD Street Fire CDI. I looked around the internet, and several people just connected the ECM to the tach signal from the CDI. Seems to be working so far. Idle is high (900 +/-), but I may need to adjust that per Howells instructions. I have about 30 miles on her so far, so maybe the computer is still learning?? 

I might decide to purchase the filter, just in case. I would rather be proactive than reactive. I just need to get a GM air cleaner assembly for the junkyard to "look smog legal" versus the open air element that Howell suggest. That will just tweak the smog techs, as the TBI conversion won't tweak them already. Again, proactive approach.

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Some of Howell's systems do have a California E.O. number, they are legal if you have the crankcase closed as required, which can include a return draw to the air cleaner and PCV.  The Howell E.O. instructions indicate the required equipment for meeting the California C.A.R.B. requirements for this system.  You're right, the visual inspection is a sure way to pass or fail, and the air cleaner needs to function as intended.

As for signaling off the ECM, my only concern would be resistance or possible damage to the ECM.  You share that others have been successful with this approach.  I'm leaning toward your reasoning and the use of the filter.

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The Air cleaner is usually the killer in visual inspection. Ironically, the California approved E.O. kit from Howell suggest an open element, which is approved by California! Its all about money and Howell paid Ca for the E.O. (proof our laws are about money and not environmental). 

Now I do have a question about idle. Disclaimer, I am at best a weekend mechanic, at that is a stretch. My Jeep wants to idle around 900-1000 RPM, and when it sits for a minute will drop down to 750-800 gradually. To me it is acting like a vacuum leak, but I have no idle change with a propane test around TBI base, vacuum ports, hoses, PCV, CTO.  There is no CE light, and I have no error codes. I am not convinced yet that I do not have a vacuum leak.

But, wouldn't a slight vacuum leak set off a code? I am concerned that I may have a slight leak though the adapter plate, which needed silicone to fill over the bolts. The plenum cuts into the cavity of the bolt heads just a little, and Howell instructions inform that the heads need to be sealed with silicone to prevent a leak.This is my initial feeling, but have not confirmed it yet.

Jeep idles around 900 rpm and is smooth, drives down the road great. When it does idle down to the 750-800 range, it does get just a little rough on idle, not much, it is just noticeable. Things I have not checked yet include: plugs/wires, manifold/ported vacuum at idle and speed, fuel pressure (in/out), voltage at ECM at idle. I did check timing and retard to Howells recommended 6 to 8 degrees with no vacuum hose attached to distributor and blocked off while setting. Distributor gets ported vacuum. It would be nice to check total timing if I had the right tools.

Anything else I should be looking at before I adjust the idle stop? I want to make sure I am not masking the problem(s).  

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Of course Howell would be the final say in engine tune and performance, but this is essentially a G.M. 4.3L Vortec V-6 TBI system.  I would troubleshoot your symptoms using a G.M. service guide for the 4.3L Vortec V-6 with TBI.  It sounds like an idle air control (IAC) issue, but if so you want to know what is triggering this problem, the devices involved and the signals to the ECM that affect idle air control or IAC functions.  

Troubleshoot the devices and also check for a manifold leak.  You can do this quickly with a can of WD-40 or a similar low volatility spray.  (Avoid spraying near heat or the exhaust manifold.)  Spray a light mist at the manifold/TBI junction with the engine at a idle.  Listen for any brief, simultaneous engine speed change.

Fuel pressure is always a place to start with EFI trouble, and the coolant temp, air intake temp and MAP signals are important, too.  Each can influence idle, but I would begin with a focus on the IAC triggers.

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I disconnected the IAC and ran the engine under load. Idle ran back up to 900-1000 again. Also heard a faint whistle sound. Vacuum leak through the TB somewhere, just haven't found it yet. The good news it is not the IAC pintle, EGR, MAP, and it is just a vacuum leak (most likely). 

Most likely place is the adapter plate. I will try some WD40 or carb cleaner this time instead of propane. 

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Latest update: EGR is bad. While that steadied the idle, still running a little high at times. Haven't gone down to get a gauge to check pressure, do that this weekend.

But, the funny thing. If I coast down the road at any given speed (clutch in and/or out of gear), she will idle around 1000 rpm. If I stop, idles down to 800 rpm +/-. If I sit still, idle will come done to 750 rpm. What the heck? Emailed Howell and I have yet to receive a reply. Phone call next.

I am new to TBI and ECM in general. To me, it is a programming issue, or speed sensor issue. I just cannot find a vacuum leak to explain the idle, nor should it vary with speed anyways. Or is there something else that could cause a fluctuation in idle according to speed?

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Nucking Futs...Yea, now you're getting somewhere with the EGR!  Does this system use a vehicle speed sensor?  The Jeep MPI conversion kit does, as the Jeep PCM relies on the vehicle speed signal for some EFI functions.  Does Howell use a VSS on the TBI conversion?  I'm not aware of it.

Check for a possible vacuum leak, including a leak between the intake manifold and cylinder head.  That gasket or your TBI-to-intake manifold could be involved.  Going down the road does affect the underhood atmospheric pressure.  Is the MAP sensor okay?  

This idle speed problem could also be unrelated to the TBI system.  You have the MSD ignition enhancement.  Does this adjust spark timing in any way or does it just enhance spark output?  If you're still using the OEM Motorcraft distributor, make sure your distributor vacuum canister is fed ported vacuum and not manifold vacuum.  Also make sure that the distributor's mechanical advance mechanism is not sticking, making it unable to smoothly and accurately retard spark timing as the engine speed drops.  A sticky centrifugal advance would hold the idle high. 

Take your timing light and watch the spark timing as the engine holds the higher rpm at an idle.  Watch the timing as the speed drops off...You could have a sticky centrifugal spark advance mechanism in the distributor, or the vacuum source for the vacuum advance might be hooked up to manifold vacuum instead of ported.  

Note: On your TBI conversion system, you can bypass the maze of 4.2L OEM spark advance/retard solenoids activated by the factory ECU.  (You can leave these devices on the chassis, but they no longer serve a purpose with the Howell TBI installed.)  Simply run a vacuum hose directly from the TBI ported vacuum to the distributor's vacuum canister...Check with a gauge to confirm that the vacuum source is ported and not manifold vacuum.  (Ported vacuum should read close to zero in/hg with the throttle valve closed.)  The TBI unit has a ported vacuum port and possibly a manifold vacuum source.  Cap off the manifold vacuum source.

There is a CTO (a coolant temperature vacuum interrupter switch) in the distributor's vacuum circuit.  This is also used for the EGR vacuum circuit.  This prevents vacuum spark advance and EGR operation until the engine reaches normal operating temperature.  I'm not sure whether Howell requires this to meet emissions standards.  If not, you can run a direct hose from the TBI ported vacuum source to the distributor's vacuum canister or run this hose via the CTO.

Be aware that EGR is also opened with ported vacuum and not manifold vacuum.  If hooked to manifold vacuum, the EGR would remain open whenever the engine is running.  The EGR valve is not supposed to open at a close throttle position or when ported vacuum drops off.  Make sure your EGR valve has a ported vacuum source.

Moses

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