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PostPosted: Fri Jan 06, 2017 1:58 pm 
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Year and Trim: 1997 Pontiac Bonneville SE
My 97 SE has a weird starting problem lately (past month or two). It only happens after waiting a short time, about 20-30 minutes after shutting off the engine. When hard starting, without touching the gas pedal, it will finally turn over after a few tries, but slowly, then run really rough for a few seconds, then smooth out to normal idle (about 800-900 RPM).

The weird thing is, it starts up OK immediately after I shut it off, and within about 10 minutes after shutting it off. However, it starts up OK if I wait much longer than 20-30 minutes, say after about two hours later (not completely cold engine). Also, I noticed that my gas mileage lately is only about 17 mpg under easy-driving conditions (about 50-50 highway-city driving).

Note that I changed my MAF (mass air flow) sensor last May (2016). It's a re-manufactured AC-Delco one that I bought from RockAuto.com for about $85 (after core credit).


Any ideas? :sad:


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PostPosted: Fri Jan 06, 2017 11:29 pm 
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Temp sensor comes to mind.

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PostPosted: Mon Jan 09, 2017 3:21 pm 
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That makes sense! THANK YOU!!!

I should have replaced my Engine Coolant Temperature (ECT) sensor when I replaced my radiator and thermostat last September. I had purchased a Delphi ECT sensor from RockAuto but I didn't replace it because I thought my old radiator (original) was bad and the new one had fixed my apparent overheating problem (approx. 220 degrees on the temp gauge) on a hot 80-90 degree NH day last Aug or Sept. However, temperature still reads a bit high when the engine is hot and standing in traffic, sometimes around 205 degrees on a warm 40-50 degree NH day in January. Apparently my old radiator was probably still good and I really only needed to replace the ECT sensor to fix my "overheating" problem. Oh well, at least I won't have to worry about replacing the radiator for another 20 years, haha.

So, I'll replace the ECT sensor and report the results, and the new gas mileage, in a couple of weeks.

Thanks again for your insight!


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PostPosted: Mon Jan 09, 2017 4:27 pm 
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There are two one is below throttle body,temp sensor,the coolant temp sending sensor is back right top of upper intake and a bit of a cluster **** to get at.The sending unit is the one that caused my car to have similar issues as yours,poor fuel mileage and hard starting.The oxygen sensor on mine was fowled from the rich condition caused by the bad coolant sending unit ,after both were replaced car ran smooth and mpg returned to normal.


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PostPosted: Thu Mar 16, 2017 9:25 pm 
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Now that it's been a couple of months since I replaced my Engine Coolant Temperature (ECT) sensor, my car is running as poor as it has been before I replaced it. Hey Pontiac1, I'll look into the other temp sensor that may be causing the problem, but I thought my ECT handled both functions, i.e. Coolant temp sensor and temp sending unit, because it has 3 wires, nut just 2. Also, My O2 sensor is fine since it's not throwing a code, so I don't have to replace it. I'll let you now what I find.


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PostPosted: Sat Mar 18, 2017 7:13 pm 
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Another possibility is a faulty fuel pressure regulator.
Pull the vacuum hose off of it and , if there is any liquid or any fuel smell, replace the FPR.


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PostPosted: Sat Jun 17, 2017 6:40 pm 
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OK MKMike, I couldn't smell any fuel at the FPR, but my sense of smell must be poor now because I can't even smell my own farts, haha. Instead, I'll try detecting the fuel smell again by asking a friend to take a whiff.

Note that 2 more events happened recently, that might shed some light on what's happening. My car has now stalled, twice over the last 2 days, for no apparent reason, after driving for about 1/2 hour. This hasn't happened before. As soon as I pulled over to the side, I cranked it a few times but it wouldn't start, keeping my foot off the gas. I also tried a little gas afterwards, but no luck. So, like before when I had waited a couple of hours to try starting it, this time I waited only 10 minutes to try again, and it successfully started (rough) right away and ran at a fast idle for a few seconds, then smoothed out to normal idle (about 800-900 RPM). I could smell some gas this time, so I know it was running rich while starting.

A second clue is that my electric windows wouldn't open or close, with the ignition on, just before I immediately tried starting the car after it stalled. However, after waiting 10 minutes, my windows did open & close just before cranking the starter. When I cranked it, the car started as noted above.

Any ideas,now that I have more information?

While waiting for an answer, I'll check the grounds, fuel pressure (need to buy a gauge), MAP & MAT sensors, and any other sensors that might have failed even though I don't have any codes being thrown.


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PostPosted: Sat Jun 24, 2017 1:57 am 
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My 97 Bonneville had a similar weird starting problem, and eventually started stalling while driving. I finally fixed it after some electrical troubleshooting.

First, let me tell you about my starting/stalling problem: After warmup while driving for a while, then shutting off and waiting less than say 10 minutes, it would startup fine. If I waited about 20 minutes it would crank hard, then finally start. If I waited about 30 minutes after shutting off, it would crank but not start at all, but then if I waited another 30 minutes it would crank hard and finally start (with a little press on the gas pedal.) Lately, it would also stall while driving for about 30 minutes, and I'd need to wait 30 minutes for it to finally start after cranking it a few times. Each time my car wouldn't start, my power windows would only move a little bit when I tried to open/close them, then stop completely. So, I had a very similar problem to yours.

At one point, before I noticed my power windows wouldn't work right, I thought it may be a fuel pressure regulator (FPR) problem. But after pulling off the FPR vacuum hose, then waited a few minutes, no gas dripped out, so my FPR was OK.

I finally found the culprit after reading some postings here! It was a single corroded wire connector in a single ground bus. Here's how I diagnosed and fixed it:

Apparently, there are two ground buses, each of them under the carpet next to the front-most side of each of the 2 front doors. I found my weird starting problem was caused by the ground bus that's next to the driver side. Specifically, it was caused by a single corroded ground wire connector that's next to the end ground wire (fattest). Note that there's an empty connector slot between these two wires/connectors. I diagnosed and isolating this problem by simply wiggling each wire (and therefore its connector too), one at a time, while the car was running. The bad wire connector caused the car to stall. Also, after it stalled, I simultaneously cranked the engine while wiggling the same wire. It eventually started when the wire was held in a particular position that made a good contact. Finally, I disconnected the wire from the ground bus and verified that my car wouldn't start. I then verified that my car would immediately start after bypassing the ground bus, only for that wire, by running a jumper from that wire directly to my battery's negative (-) terminal. I took apart my ground bus, used a wire brush on the bus and connectors, then packed them with dielectric grease.

Note that using a voltmeter to diagnose this type of intermittent problem (with the engine off, and then testing again with it running) is not a good approach, because pressing on the wire's connector with a voltmeter probe may actually force the wire's connector against the ground bus bar, thus making good contact. This is what I had initially tried at first, with my engine shut off, but couldn't find any problem using my voltmeter. Luckily, my car stalled while I was testing the bus with my voltmeter while the engine was running!

Other possible locations of corroded ground wire connections are:
1) Rear of the engine block near the oil pressure switch.
2) Left front of the engine compartment near the air cleaner.
3) Bolted ground connection under the glove box by the front of the door frame.
4) Grounds to the block under the Ignition Control Module (ICM), located under hood, mounted on upper front passenger side of engine.


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PostPosted: Thu Jul 06, 2017 4:39 pm 
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OK guys, here's what I found about my weird starting & stalling problems:

I actually have 2 problems that have been giving me headaches for a while, but I think I fixed one of them - - the stalling problem that also caused my windows not to close (or close very slowly). That was the single corroded ground wire terminal in the ground bus under the carpet next to the front-most side of driver door. However, I still have the weird startup problem that only occurs about 20 minutes after I shut off a warm engine. I can barely get it started if I wait a few minutes and pump the gas a bit while the engine cranks.

So far, I've replaced my MAF (Mass Air Flow) sensor and Engine Coolant Temperature (ECT) sensor. I also tested the fuel pressure regulator by pulling its vacuum hose and waiting 5 minutes to see if any gas leaked out (and gave it the smell test). I don't think any of these are the culprit now.

Still no codes are being thrown, so I tested (per my Haynes repair manual) the IAT (Intake Air Temperature) sensor) [a.k.a. MAT (Manifold Air Temperature) sensor in older Bonneville models] which is probably what Pontiac1 is referring to (a posting above) as the one "below throttle body,temp sensor". So, I removed the IAT and connected it to a digital ohm meter and it measured 2.4 kohms at 77 degrees F. Then while I heated it up with a ceramic room heater to about 200 degrees F, it changed to about 300 ohms. So, apparently it's internal thermister is working, although my Haynes manual said it shouldn't change by more than 500 ohms, but probably expecting no more than a 50 degree change in temperature. Also, the Autozone online manual said it should change from about 24 kohms to about 3 kohms over the wide temp range that I submitted it to. So maybe the decimal point in Autozone's table is in the wrong place. Anyways, it appears to be working properly to decrease its resistance with an increase in temperature. Also, its connector wires are OK because I'm reading 5VDC with the ignition on, and after I start the engine.

So, I pretty much ruled out the MAF, ECT, IAT and fuel pressure regulator.

Any more ideas?


Last edited by StraTact on Thu Jul 06, 2017 6:50 pm, edited 1 time in total.

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PostPosted: Thu Jul 06, 2017 6:41 pm 
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I have heard of Crank Position failures causing similar issues, but typically, when they go, they just quit working until they cool back down, so the car will crank and crank, but never fire. This is usually diagnosed by pouring cold water on the sensor when the symptoms are present, and the car will start. There will also be no tach signal, as well as no fueling or spark when this is happening too.

They also very rarely throw codes. The computer just sees the signal disappear and assumes the engine stopped.

Also, ICM's can be very fiddly when they start to go out, and also very rarely throw codes when they go.

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PostPosted: Fri Jul 07, 2017 12:12 am 
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please unplug the maf just to rule out defective parts

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PostPosted: Sat Jul 08, 2017 3:28 pm 
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Year and Trim: 1997 Pontiac Bonneville SE
Thanks for your feedback, RJolly87 and 96 SSEi. I'll try your suggestions and also try those from another forum, https://www.cargurus.com/Cars/Discussio ... 0_ds536438 , which are very very helpful.

I'll post my findings as I continue testing various sensors, etc.


Last edited by StraTact on Sat Jul 08, 2017 10:05 pm, edited 1 time in total.

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PostPosted: Sat Jul 08, 2017 10:04 pm 
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Here's what I just tried:

I removed the gas cap before starting the car, and it started OK. So I ruled out the gas-tank pressure leaks.

I unplugged the MAF, and started the car after a few tries, but I had to give it some gas, around 2000 RPM to keep it running. When I took my foot off the gas after a minute or so, it stalled. I think this was all normal because when I reconnected the MAF and started it up, it ran a bit rough for about 10 seconds, and then smoothed out to around 900 RPM and ran quiet. Like I said earlier, I had ruled out the MAF because it was a recent remanufactured one that I had recently cleaned with MAF cleaner, and the engine runs fine, with no hesitation, when I accelerate.

I wiggled the CPS' wires while the engine was running, and it continued running smoothly, so I ruled out its wire connections being loose or corroded. But I haven't tested the CPS yet by spraying water on it when the engine won't start. I just have to wait for it to not start again after it's warmed up. Also, I haven't yet tested the TPS either, and don't know whether it can cause the problem I'm having, but I'll test it anyways.

I'll continue with various tests and let y'all know what I find.


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PostPosted: Tue Sep 12, 2017 3:08 pm 
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Hi, I'm back again with more results, after replacing my Crankshaft Position Sensor (CPS) and testing my engine's engine's Vacuum and Fuel Pressure.

My new CPS didn't solve any problems, and I have some strange fuel pressure results after taking some measurements.

First, I forgot to mention the problem I have with engine speed oscillations (about once per second) only while driving at about 60 mph and 2200 rpm during a long gradual incline. Also, I'm continuing to have starting problems (needed to pump gas, then wait a minute, then try again and again, but then it starts). This usually happens a half-hour after I shut off the engine to do some shopping. As noted in previous postings, I replaced my MAF sensor (ACDelco 2133423 [remanufactured]) and I've recently replaced my Crankshaft Position Sensor (CPS) (ACDelco 213-151), but they haven't helped for any of the above problems.

Yesterday, I decided to test my engine's Vacuum and Fuel Pressure. My vacuum seemed OK, but my fuel pressure was kinda strange. Here's what I found, along with some findings from my Haynes Repair Manual:

My vacuum specification is 17 - 22 in. of vacuum, at idle. I measured 18 inches of vacuum, within spec., directly at my Fuel Pressure Regulator, using a "T" hose connection and a Sun engine vacuum gauge.

My Fuel pressure specification in my Haynes Repair Manual is 40-47 p.s.i., with ignition on but engine not cranking, and also while engine is running. My Haynes Repair Manual tell me that if my fuel pressure is too low, it could be a blocked fuel filter, bad fuel pressure regulator, bad fuel pump, or some combination. My manual also says that is my fuel pressure is too high, it could be that the fuel return line is blocked.

Anyways, I measured the Fuel Pressure before the fuel filter, right at the tank's output, using a "T" hose connection and a Fuel Pressure Gauge kit I borrowed from Auto Zone. Here's a table of results:

Fuel Pressure: Condition
?? p.s.i.: Turned ignition on, before cranking engine. (I forgot to note this measurement, but I did so later at measurement I took at the Schrader fitting on the fuel rail).
40 p.s.i.: Started Engine from cold state. Steady reading.
40 p.s.i.: Over the next 30 minutes and steady reading.

So, it appears that the fuel pressure at the output of my Bonnie's fuel pump is borderline. This might explain oscillating hesitation (about once a second) when my engine was calling for more fuel during a long uphill climb at 60mph and about 2200 rpm.

Next, I measured Fuel Pressure at the Schrader fitting on the fuel rail: Here's a table of results:

Fuel Pressure: Condition
28 p.s.i.: Engine Off and cold, after sitting overnight.
38 p.s.i.: Turned ignition on, before cranking engine.
40 p.s.i.: Cranked/Started Engine. Took a few seconds afterwards to reach this, but engine cold.
42 p.s.i.: Engine running for about 10 minutes, and about half-way hot.
40-44 p.s.i.: Oscillating rapidly when engine became hot after another 5 minutes.
42 p.s.i.: Engine Shut Off. Reached this pressure immediately.
50 p.s.i.: 2 minutes after engine shut off. Climbed rapidly to this value, with engine still shut off.
50 p.s.i.: 5 minutes after engine shut off.
48 p.s.i.: 25 minutes after engine shut off.
45 p.s.i.: 30 minutes after engine shut off.
41 p.s.i.: 35 minutes after engine shut off.
28: 40 minutes after engine shut off. Engine warm, not hot.
40-44 p.s.i.: 45 minutes after engine shut off. Engine warm. Started engine, and pressure oscillated rapidly.
42 p.s.i.: Engine Shut Off. Reached this pressure immediately.

My higher-than-normal fuel pressure with the engine shut off might be caused by my fuel return line being blocked somehow, probably from deterioration on the inside of the steel return line. I'm not sure about the fuel pressure oscillation between 40-44 psi. Maybe the oscillations are caused by a faulty fuel injector?

Any thoughts or suggestions?


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PostPosted: Sat Oct 07, 2017 1:20 pm 
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Actually, my Haynes Repair Manual might be wrong about my 97 Bonneville fuel pump pressure. It says 40-47 psi, but typical pump specs for it are 51 psi.

So, I'm not sure if my pump is borderline or way too low.

In any case, I'm going to replace my tank, pump, sending unit, etc because after 165K miles it's about time I did it anyways. It should run better afterwards.

I'll let you all know what happens.


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PostPosted: Thu Oct 12, 2017 10:11 am 
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I fought a similar issue as this for two years, hopefully you have not dropped the tank yet...

Use a DMM to check the resistance on the primary side and secondary side of each coil. The techinfo section of the forum has the resistance each coil should have. powertrain-ignition-sensors-etc/topic1683.html

So 0.5ohm on the primary and 6.5K ohm on the secondary.

I had a weak coil cause hard starting or misfiring when first started up in the same situation as yours. The only other symptom I had was a intermittent misfire under boost at WOT. A weak coil is a coil where part of the insulator of the winding has overheated and crossed another wire in the winding, which reduces the number of turns in the electrical path and therefore less spark potential capability.

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