TBI rough on a warm or hot restart

[r67northern]

Hi All, I'm looking for some ideas on this one.

The 454 in my R20 is running great, I've finally chased down all of the random stuff that it needed after I found it in Texas a couple years ago.

Now I'm just left with this problem: When it's hot, and turned off for a little while, it restarts great for about 10 seconds and then goes into a very rough idle and dies. It's a bit sporadic, but it's only when it's still warm. It hates restarting once this happens and you've got to keep your foot in it to keep it from dieing - and it will be rough for about a half minute until it smooths out again and is fine.

I have been able to overcome the problem by turning the ignition on/off a couple of times at which point it restarts fine again. And I've heard the fuel pump solenoid kick on like it should and then still have it go into the rough idle anyway.

Is this a fuel priming problem? Or a temperature sender problem? Any other ideas?

Thanks from Louisiana and Merry Christmas.

 

[MrMarty51]

Could be the temp sensor or the O2 sensor.

 

[1987 GMC Jimmy]

I'd like to know what the fuel pressure is coming from the pump. Also, have you changed the fuel filter? The fact that you're priming it multiple times, and it'll start up makes it sound like weak pressure. I wouldn't immediately suspect any sensors unless it ran super rich all the time. That being said, I wouldn't discount the CTS, but there are only two ways to know for sure on that one. Either replace it and see what happens or get a scan tool to see if it's always telling the ECM the engine is ice cold. Same with the O2 sensor. I've found that it'll tell you most often about an O2 sensor or mixture issue than anything else. No codes? Do you have good quality spark, especially when it won't start, and a good timing setting? I'd check your spark quality/presence and timing just to eliminate that from consideration. I'd then borrow a fuel pressure tester from the store and get a new filter if I haven't replaced it and check all that out. You should see 12-14 pounds. I wouldn't be confident in less than that. If both of those check out good, I'd move to the sensors, but if you're not getting codes, you really need a scan tool to see what the sensors are doing without throwing parts at it. A homemade scan tool shouldn't cost more than $7 to make, and there's a sticky about it in the TBI/EFI subforum.

 

[r67northern]

Thanks for the advice guys, and no, there are no codes.

I'll start running through the list of options starting with fuel pressure and spark timing. I've never done the fuel filter so that's a no brainer to go after regardless. How do I tell if I have a good quality spark? Under load there's no hesitation so I've been assuming it's good.

Also - on my Roadmaster it's practically impossible to get an easy fuel pressure check because of the way it's all plumbed in. Same true for these truck TBIs ?

O2 sensor is a good thought too, that's another one that I've never replaced but I'll run through the fuel/spark first.

 

[1987 GMC Jimmy]

A good quality spark is a white one when you ground the plug and turn the engine over. A poor one would be yellow. I mostly suggested this to verify you're getting good spark when you have your issue. To test your fuel pressure, you're gonna have to tee in your tester where the filter goes. Either that or buy Actron adapter that goes behind the TBI to do it there.

 

[gpmorgan]

I had a similar problem with a 350 tbi. It was the fuel pump relay. Took it off and opened it up and found the contact tips burnt in the relay. Sanded and cleaned the contact tips and ran it for years. It would run ok but you had to spin the motor for several seconds before it would start.

 

[r67northern]

Thanks all - much appreciated

 

[r67northern]

Hi All - bringing this thread back with an update.

I crawled under and replaced the fuel filter, did a fuel pressure check at the same time. It's running about 12ish, I guess I'd rather have 14+ but I guess that's okay? The thing that confuses me is that I thought the pump was supposed to cycle when the ignition is on, but engine not running.

I turned on the ignition (engine off), pump wound up, pressure hit about 12, then nothing ... ever. Pressure drops back to 0. Engine started, pump back on and running at about 12 again.

Thoughts? I swear my TBI wagon cycles the pump to maintain pressure in the ignition 'on' position.

 

[Snoots]

Not sure about TBI but DI only brings up operating pressure once when the ignition is in the 'on' position. It does not cycle to maintain that pressure if the engine is not running.

 

[r67northern]

Not sure about TBI but DI only brings up operating pressure once when the ignition is in the 'on' position. It does not cycle to maintain that pressure if the engine is not running.

Well, not the first time I've been wrong. Today.

 

[Snoots]

Well, not the first time I've been wrong. Today.

I didn't say you were wrong. You may have been mistaken and then again, I may be too.

 

[r67northern]

I didn't say you were wrong. You may have been mistaken and then again, I may be too.

Maybe having a wide and kid makes me too quick to assume I'm just wrong

 

[Snoots]

Maybe having a wide and kid makes me too quick to assume I'm just wrong

The wife and kid are always wrong. You just NEVER tell the wife.

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