1976 Seville ECU problem, Hard Cold Start

I removed the Fast idle 'heater' from the throttle body and checked it. That was okay.

In fact, when I do get it started the fast idle is about 1200 rpm, and over 5 or more minutes, depending on outside temperature and throttle body temperature, the fast idle slows to normal speed of 650 rpm,

So, the fast idle bypass which is controlled by the fast idle heater is okay, in my opinion.

The engine starts fine when warm from operation.

The engine does not start immediately when not run for a few hours to overnight not run.

I believe the problem is related to the computer, particularly the circuit that enriches the fuel by increasing the length of time the injectors open when the starter solenoid is engaged, when you crank the engine, which happens when cold like 45-60 degrees F.

Note. I tried three different ECU computers, and so far each acts differently when starting cold.

Once started, after several cranking attempts,it runs fine at all speeds, and also starts up fine, with the first ECU I got from Blue Streak, at least.

Duane Wong

Duanewong said:

I priced a used fast idle bypass heater on eBay. It was $235 to $305 or so, which is almost the price of an ECU computer.

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I know what you are saying about the idle air control. I changed the IACV on my Northstar engine in a 1997 cadillac deville limousine, twice.

The 'fast idle heater' just extends a pintle that closes throttle body air bypass that makes the engine run faster when started cold, combined with wider pulse width of the fuel injectors when the engine coolant temperature sensor is showing cold coolant.

It is like open loop on later engines that use an oxygen sensor.

Duane Wong

cadillac_al2 said:

It's probably your idle air control, those were a common failure point. I think it may have had a different name back then. It just twists out of the throttle body and is probably hard to find now.

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Fast Idle Heater does twist out, and I used throttle body cleaner spray on metal throttle plates.

It appeared clean before and after.

It had no effect on cold hard starting problem.

Hello, It's almost certain you have a bad coolant temp sensor. They are very easy to check. There are actually two sensors which feed information through their resistance to the ECU. At 60-70F they should both read roughly between 950 and 1000 Ohms. If it's cold out and especially the coolant temp sensor fails with open circuit, the most common, you will not be able to start he vehicle. I had the same issue and nothing was available so I did a lot of research and I've been making professional replacements with the original factory 2 pin connectors for a little over 2 years now. I just joined this site but I am a frequent contributor on the Cadillac site I found prior to finding this site. Check yours before you buy anything; easy with an Ohm meter. The replacement PN is 1609968.

If the ECU is bad there is a member on *************** who rebuilds them. I would 1000% recommend sending yours to him if you still have an issue which you think is ECU related but there are some simple things you can check first. If the issue is cold start though the diagnostic points you to check coolant sensor first (total of 3 coolant sensors on you car so you need to check the 2 wire sensor) and then second check the air temp sensor located normally behind the throttle body on the intake (same resistance I mentioned above).

As for the FIV Fast Idle Valve it controls bypass during cold start and results in fast idle followed by closing after warming. That PN varies but after 2 years of working on them I have finally created a plug-and-play replacement for those as well. The PN are as follows: GM PN 1628898, GM PN 1617956, GM PN 606776and is also known as Bendix PN 1181290 and Bendix PN 1181835 My replacement is the cream/ natural colored one found on eBay but I sell both the sensors and FIV's to forum members at a discount on that other forum.

Best regards,
Scott
PS: Someone is already using the forum name I use on *************** but I am "79 Eldorado" there and "Punched455" on eBay.

1979 Eldorado said:

Hello, It's almost certain you have a bad coolant temp sensor. They are very easy to check. There are actually two sensors which feed information through their resistance to the ECU. At 60-70F they should both read roughly between 950 and 1000 Ohms. If it's cold out and especially the coolant temp sensor fails with open circuit, the most common, you will not be able to start he vehicle. I had the same issue and nothing was available so I did a lot of research and I've been making professional replacements with the original factory 2 pin connectors for a little over 2 years now. I just joined this site but I am a frequent contributor on the Cadillac site I found prior to finding this site. Check yours before you buy anything; easy with an Ohm meter. The replacement PN is 1609968.

If the ECU is bad there is a member on *** who rebuilds them. I would 1000% recommend sending yours to him if you still have an issue which you think is ECU related but there are some simple things you can check first. If the issue is cold start though the diagnostic points you to check coolant sensor first (total of 3 coolant sensors on you car so you need to check the 2 wire sensor) and then second check the air temp sensor located normally behind the throttle body on the intake (same resistance I mentioned above).

As for the FIV Fast Idle Valve it controls bypass during cold start and results in fast idle followed by closing after warming. That PN varies but after 2 years of working on them I have finally created a plug-and-play replacement for those as well. The PN are as follows: GM PN 1628898, GM PN 1617956, GM PN 606776and is also known as Bendix PN 1181290 and Bendix PN 1181835 My replacement is the cream/ natural colored one found on eBay but I sell both the sensors and FIV's to forum members at a discount on that other forum.

Best regards,
Scott
PS: Someone is already using the forum name I use on *** but I am "79 Eldorado" there and "Punched455" on eBay.
View attachment 593399 View attachment 593400

Click to expand...

1979 Eldorado said:

Hello, It's almost certain you have a bad coolant temp sensor. They are very easy to check. There are actually two sensors which feed information through their resistance to the ECU. At 60-70F they should both read roughly between 950 and 1000 Ohms. If it's cold out and especially the coolant temp sensor fails with open circuit, the most common, you will not be able to start he vehicle. I had the same issue and nothing was available so I did a lot of research and I've been making professional replacements with the original factory 2 pin connectors for a little over 2 years now. I just joined this site but I am a frequent contributor on the Cadillac site I found prior to finding this site. Check yours before you buy anything; easy with an Ohm meter. The replacement PN is 1609968.

If the ECU is bad there is a member on *** who rebuilds them. I would 1000% recommend sending yours to him if you still have an issue which you think is ECU related but there are some simple things you can check first. If the issue is cold start though the diagnostic points you to check coolant sensor first (total of 3 coolant sensors on you car so you need to check the 2 wire sensor) and then second check the air temp sensor located normally behind the throttle body on the intake (same resistance I mentioned above).

As for the FIV Fast Idle Valve it controls bypass during cold start and results in fast idle followed by closing after warming. That PN varies but after 2 years of working on them I have finally created a plug-and-play replacement for those as well. The PN are as follows: GM PN 1628898, GM PN 1617956, GM PN 606776and is also known as Bendix PN 1181290 and Bendix PN 1181835 My replacement is the cream/ natural colored one found on eBay but I sell both the sensors and FIV's to forum members at a discount on that other forum.

Best regards,
Scott
PS: Someone is already using the forum name I use on *** but I am "79 Eldorado" there and "Punched455" on eBay.
View attachment 593399 View attachment 593400

Click to expand...

That's neat you have parts for repairing common problems found on the 1976 Cadillac Seville.

The way it acts, though, is almost like it is fuel starved when initially cranking the engine for several seconds.

Then, if you just tap the starter switch it starts up immediately.

And, when the engine is run a couple of minutes to an hour or longer, the car restarts normally.

I tested the ohms of the coolant temperature sensor, which is the same part # as the intake air temperature sensor, but they were both within specs.

The car was at a dealership and they tested the temperature sensors and what you call the FIV Fast Idle Valve, which were found to be functioning normally.

Thanks for your thoughts on the problems, though.

You sure this is not a low fuel pressure on cold start ?
Have you measured and seen if the PSI is around 45 PSI ?

Have you done a leak down test by once warmed up measure fuel pressure then with hood up shut down
and then record how long before pressure drops and what that pressure is ?
How old is the fuel filter, possible clogged reducing PSI or volume ?

I assume the GM ECM calibration is within a E-prom and if so Have you switched around those
in your different ECMs and seen if one ECM functions better with a certain one of those E-proms

Have you looked closely for any type of air or exhaust leak ?

What does this engine have, multi fuel injectors or some MFI type carb ?
Measure that air temp heater is getting hot enough ?

38-40 PSI fuel rail pressure seems low and at what temp or RPM is that ?

Is there a fuel regulator, and is it vacuum controlled, if so check that hose of leak ?
Is regulator adjustable, and if so what happens if railing fuel rail PSI to more like 45 PSI ?

When starting cold and idle is high about 1,200 RPMs something is driving ECM as to timing or
what it command to the fuel injector pulse width ON times

Check vacuum, what is reading when cold started
Do leakdown of fuel pressure, with 8 injectors must have a fuel rail and it should have a valve to
screw on a fuel pressure tester to assure it is not what the issue is and also assure the regulator is
working correctly or adjusted for proper PSI

If there is some type of intake leak which will effect vacuum then ECM may be fooled as to what engine mode the ECM is

If fuel system and injectors not a issue then how about getting or making a smoke machine and looking for small leaks

I think the fuel pressures were lower on the first generation fuel injection specifications in the 1976 Cadillac Seville compared to my 1990 Cadillac Deville where it was 10-15 psi, or so, higher.

I'd have to look at the invoice to be sure of the exact pressure noted.

68° F to 75° F last summer.

Ignition on, starter off.

I am not sure I'd the technician looked at the pressures while cranking it, or while it was running.

I think he had only one figure written down.

Vacuum controlled fuel pressure regulator was changed, too, last summer.

The thing that gets me is every ECU I have tried has different behaviors for starting the engine. Some worse some better.

One ecu flooded the engine after turning it off.

Another, would not start the car at all.

And, the one in it now, has hard cold starting, but if you try long enough it starts and runs flawlessly after that especially when warm.

Duane Wong

Looking at the website you point to it shows what looks like max fuel rail pressure is
you could do the test they stated :

7. Since diagnosis this far has failed to find a reason why the fuel pressure is low, begin to suspect that the pump or the regulator may be the problem.
With the engine at idle, pinch off the fuel return hose. This action is how the regulator raises the rail pressure.
If the pressure does not increase to at least 46PSIG, a defective chassis pump is indicated.

Also I see this has a MAP sensor, check for a hose leak/vacuum or test to see if MAP is faulty when cold
No way of knowing with those ECMs if they in fact were setup for your engine, model or year

Get tests out of the way if it is a fuel, MAP or intake leak, using the diag flow of that website you use

MAP voltage values should pin down if there is some vacuum leak (inverse of MAP) or again use a smoker